gh-35749: Add style guide / reference for `# optional - sage....` doctest tags, extend `sage -t` and `sage -fixdoctests` for modularization tasks
    
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### 📚 Description

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- Fixing the handling of file-level `# optional` tags.
- Some files were not being doctested; fixing the recently introduced
errors in doctests.
- Implementing block-scoped tags (originally done in PR #35779, merged
here)
- Expanding the documentation on the `# optional` / `# needs` tags used
for modularization purposes, with examples.
- Adding features `sage.modular`, `sage.numerical.mip`,
`sage.rings.complex_double`, `sage.sat`
- The tools `sage -t` and `sage --fixdoctests` receive some new options
for modularization tasks (see added documentation):
   ```
   $ make pypi-wheels
   $ make SAGE_CHECK=yes sagemath-modules
   $ ./sage --fixdoctests --distribution sagemath-modules \
src/sage/combinat/root_system/root_lattice_realization_algebras.py
   ```
   (example uses #35095)
- Suppressing `# optional`/`# needs` of present features in the help
system
.

<!-- Why is this change required? What problem does it solve? -->
Motivated by the sage-devel thread https://groups.google.com/g/sage-
devel/c/utA0N1it0Eo (2023-06)

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example "Fixes #12345". -->
- Resolves #35790
- Resolves #35750
- Part of #29705
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appropriately. -->

### 📝 Checklist

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- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.

### ⌛ Dependencies

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- #34567: ...
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- Depends on #35820 (merged here)
- Vote at https://groups.google.com/g/sage-devel/c/8KZNRBs6F6U (result:
https://groups.google.com/g/sage-devel/c/MtS2u3VbJEo)
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URL: #35749
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

sagemathgh-36741:  Doctest hide option: Better detection of hidden packages
    
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This PR implements the suggestion made in
sagemath#36696 (comment).
This means that it introduces a counter in the feature class to detect
the number of events a feature has been asked to be present while it was
hidden. This allows to remove the call of the `is_present` method in
`doctest/control.py`.

In order to test it I ran `sage -tp --all --hide=all`. Ideally this
should give *All tests passed*. But I got two failing files. One of
those was `src/sage/features/databases.py` because
`database_cremona_mini_ellcurve` was detected to be optional even
thought it is a standard package. This is a leftover of
sagemath#35820 which I fix here.

Similarily I got 37 failures in
`src/sage/groups/abelian_gps/abelian_group.py` since
`gap_package_polycyclic` was classified optional even though it is a Gap
standard package since Gap 4.10 (as well as `gap_package_atlasrep`). But
in this case a corresponding fix, i.e.:

```
 def all_features():
-    return [GapPackage("atlasrep", spkg="gap_packages"),
+    return [GapPackage("atlasrep", spkg="gap_packages",
type='standard'),
             GapPackage("design", spkg="gap_packages"),
             GapPackage("grape", spkg="gap_packages"),
             GapPackage("guava", spkg="gap_packages"),
             GapPackage("hap", spkg="gap_packages"),
-            GapPackage("polycyclic", spkg="gap_packages"),
+            GapPackage("polycyclic", spkg="gap_packages",
type='standard'),
             GapPackage("qpa", spkg="gap_packages"),
             GapPackage("quagroup", spkg="gap_packages")]
```

is not the correct way (it gives `UserWarning: Feature
gap_package_polycyclic is declared standard, but it is provided by
gap_packages, which is declared optional in SAGE_ROOT/build/pkgs`). I
would say, the correct fix would be to remove both Gap packages from the
`all_features` list and erase the corresponding *optional tags*  in
`permgroup_named.py`, `distance_regular.pyx`, `abelian_aut.py`,
`abelian_group.py` and `abelian_group_gap.py`. If you agree I will open
another PR for this.


BTW: there seem to be more packages with ambiguous type (detected with
current Docker image):

```
Digest:
sha256:790197bb223bd7e20b0a2e055aa7b4c5846dc86d94b2425cd233cb6160a5b944
Status: Downloaded newer image for sagemath/sagemath:develop
┌────────────────────────────────────────────────────────────────────┐
│ SageMath version 10.2.rc4, Release Date: 2023-11-17                │
│ Using Python 3.11.1. Type "help()" for help.                       │
└────────────────────────────────────────────────────────────────────┘
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃ Warning: this is a prerelease version, and it may be unstable.     ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛
sage: from sage.features.all import all_features
sage: [(f, f._spkg_type()) for f in all_features() if f.is_present() and
f._spkg_type() != 'standard']
[(Feature('sage.misc.cython'), 'optional'),
 (Feature('database_cremona_mini_ellcurve': Cremona's database of
elliptic curves),
  'optional'),
 (Feature('gap_package_atlasrep'), 'optional'),
 (Feature('gap_package_polycyclic'), 'optional'),
 (Feature('sage.libs.ecl'), 'optional'),
 (Feature('sage.libs.gap'), 'optional'),
 (Feature('sage.libs.singular'), 'optional'),
 (Feature('sage.numerical.mip'), 'optional')]
sage:
sage: [(f, f._spkg_type()) for f in all_features() if not f.is_present()
and f._spkg_type() == 'standard']
[(Feature('sagemath_doc_html'), 'standard'), (Feature('sage.sat'),
'standard')]
```


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URL: sagemath#36741
Reported by: Sebastian Oehms
Reviewer(s): Matthias Köppe, Sebastian Oehms