docs: contrib Delta design docs + user guide [Delta contrib split, part 8] - #12

…rt 8]

Adds the Delta contrib documentation: the user-facing guide plus the in-repo
design docs. Docs only -- no code.

User guide (docs/source/user-guide/latest/):
- delta.md (new): how to build with -Pcontrib-delta + --features contrib-delta,
  the supported Spark/Delta/Scala matrix, usage, the four tuning configs (verified
  against DeltaConf.scala), supported features, and current limitations.
- datasources.md, index.rst: link the new Delta guide into the data-sources page
  and the user-guide toctree (additive).

Design docs (contrib/delta/docs/, 12 files): overview, planning, native execution,
design decisions, build/deploy, fallback/ops, Spark 3.5 feasibility, known
limitations, plus the iceberg-style kernel-read migration plan and its
coherence/elimination audits. These are internal architecture/history docs linked
from delta.md via GitHub URLs.

Audited every config/class/path/proto reference and every user-facing claim against
what actually landed (docs were authored for the integration branch, which this
split reconstructs). Accuracy fixes:
- delta.md storage: add Azure (abfs/abfss/wasb) and GCS (gs) -- both ship and work
  via object_store::parse_url (engine.rs); the line previously listed only local /
  HDFS / S3.
- delta.md limitations: the residual S3-credential gap is explicit Hadoop
  credential-provider classes (AssumedRole/WebIdentity), NOT "per-bucket chains"
  (per-bucket static keys are handled); add the narrow far-future (~year 2262)
  INT96-timestamp overflow caveat (delta-kernel gap, A6).
- delta.md: Java 17 is required for all Spark 4.x builds (4.0 and 4.1), not just
  4.1; note Scala 2.12 is offered only for Spark 3.5.
- delta.md usage: clarify the comet-spark jar must be the from-source -Pcontrib-delta
  build (the published Maven artifact carries no Delta support).
- 12-elimination-evaluation.md: the proto kernel_read (field 25) row said "kept";
  it is `reserved 25` and planner.rs no longer reads it -- corrected to "removed".
- 05-build-and-deploy.md: `cargo build -p comet` -> `-p datafusion-comet`.

The four tuning configs (hand-documented because contrib configs are not in the
default GenerateDocs output), the version matrix, the native module list, the proto
messages, and all inter-doc links verified accurate against HEAD. Docs 10/11/12 are
explicitly framed as historical (doc 10 has a "Status: IMPLEMENTED and default"
banner).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01BtErWgRQKCDRAg8Mk6qR4G

Archived the three point-in-time engineering records (the iceberg-style kernel-read
migration plan, its design-coherence audit, and the custom-code elimination
evaluation) under contrib/delta/docs/archive/, leaving 01-08 + README as the living
doc set. Incoming links (01/03/04/README) repointed to archive/; the README gains an
"Archived design history" note. They document how the design was reasoned/pruned, not
how the shipped integration works.