Summary

This cross-repository merge unifies the documentation, benchmarks and code of conduct from the metapackage into Qiskit/Terra's build.

There are very non-trivial merge conflicts that have been resolved by this commit. The summary is:

• CODE_OF_CONDUCT.md: taken directly from the metapackage's version.
• docs/conf.py: strongly unified, albeit without the translations components that are added in a separate commit.
• docs/index.rst: taken almost verbatim from the metapackage. All the API documentation RST files on Terra are moved to docs/apidoc (without the trailing 's') to match the metapackage expectation, so the URLs of built documentation will not change.
• docs/release_notes.rst: The metapackage's version is renamed to docs/legacy_release_notes.rst, given a small introductory header, and made an orphan linked only from a new docs/release_notes.rst that uses reno.
• docs/tutorials.rst: Mostly these were the same already. Updated to include Qiskit/Terra's correction that it's not an orphan, and contain the metapackage's extra intro tutorial.
• docs/apidoc/terra.rst: renamed to docs/apidoc/index.rst and retitled to be correctly just "API Documentation".
• requirements-dev.txt: the version of the Sphinx theme is bumped to 1.14 to match the metapackage expectation.

Following merge commit ec5c9ca, there is a commit 66a5d9f that fixes the ASV build for use in Terra, which is the rollup of the post-merge commits of #10546, which this PR supersedes.

This should probably be merged ASAP before main moves on.

Resolutions on the metapackage:

• Fix Epic - Move benchmark code out of metapackage qiskit-metapackage#1723
• Fix Epic - Copy documents from qiskit/qiskit-metapackage to qiskit/qiskit-terra qiskit-metapackage#1722
• Fix [blocked] Terra's How To and Explanations docs not hooked up qiskit-metapackage#1746

After this has merged, #10610 should merge which will close the remaining migration-related issues from the metapackage.

The metapackage was prepared for the migration using git-filter-repo, with the scripts and configuration files contained within
metapackage_migration.zip. If you extract that zip, you need to activate a Python 3.11 venv then run metapackage_rewrite.bash which will prepare the repo in the exact state I merged to create this PR.

edit: In retrospect writing this, you might need to modify my script so that it pulls only starting from commit
Qiskit/qiskit-metapackage@f131daf, which was the tip of master at the time I ran this.

I tested the docs build locally and it looks as correct as I can tell. There's still big cards pointing to experiments, dynamics etc on the landing page, but I figured that enough's enough, and we can just fix those last two things in Terra. This PR does not include Qiskit/qiskit-metapackage#1791, which should be now cherry-picked onto Terra.

Details and comments

(cherry picked from commit 965fd23)
Backported from #10611

This commit is a squashed version of the merge commit from #10611. For the stable branch we don't need to preserve the full commit history like we did on the main branch. The contents of the PR branch here match the diff of #10611 but doesn't include the full 250 commits as it's a squashed cherry-pick of the merge commit. This was done manually because the mergify bot is unable to cherry-pick a merge commit. A merge commit was used for the PR to main to preserve the full commit history for the migration from qiskit-metapackage.

The one merge conflict was the because docs/apidoc/passmanager.rst which doesn't exist on the stable branch.