Migrate off `rst` files

[natestemen]

As discussed in https://github.com/unitaryfund/mitiq/blob/master/docs/CONTRIBUTING_DOCS.md#updating-the-documentation, we wish to part ways with rst. We only have the following 5 files left.

• [x] docs/source/apidoc.rst
• [x] docs/source/release.rst (migrated in https://github.com/unitaryfund/mitiq/pull/1404)
• [x] docs/source/zz_bibliography.rst
• [x] docs/source/guide/error-mitigation.rst (migrated in https://github.com/unitaryfund/mitiq/pull/1404)
• [x] docs/source/guide/_guide-zne-lindblad.rst (removed in https://github.com/unitaryfund/mitiq/pull/1494)

Once these are all migrated, the aforementioned section of the docs should also be removed.

[natestemen]

This line can also be removed: https://github.com/unitaryfund/mitiq/blob/a48f35f9ba7fab2f491c2baac2a905b6f3706b40/docs/source/conf.py#L100