Part of #22. 

We still need to productionize these files: 

1) convert RST to MDX
2) edit them to ensure they're ready

But this at least gets the files in the right locations, while keeping
their Git history intact. Once they are ready to go live, we will remove
the `__` prefix from the file name and add the file to `_toc.json`.

Some files are left behind in qiskit-docs:
- 3 tutorials, which are handled in
#125
- `faq.rst`, which we're still trying to figure out where to put

Part of Qiskit#22. 

We still need to productionize these files: 

1) convert RST to MDX
2) edit them to ensure they're ready

But this at least gets the files in the right locations, while keeping
their Git history intact. Once they are ready to go live, we will remove
the `__` prefix from the file name and add the file to `_toc.json`.

Some files are left behind in qiskit-docs:
- 3 tutorials, which are handled in
Qiskit#125
- `faq.rst`, which we're still trying to figure out where to put

This improves upon Qiskit#70.
These files were basically copy and pasted from open source, so we want
to bring in the proper Git history for them.

This PR is split into four commits—and we won't squash and merge—so that
Git properly detects how the files have changed.

This update also results in us bringing in some improvements to
operators_overview that had been made in open source, like fixing typos.