🧪🤖 Integrate RTD w/ the tox workflow tool natively

[webknjaz]

This change wires all the Sphinx invocations in RTD via tox, making it rely on the same centralized automation that contributors would use locally and GHA may run as CI.

To achieve that, the PR includes a few additional behind-the-scenes improvements borrowed from other projects I co-maintain and occasionally sync infra across (like ansible/awx-plugins et al).

The installed package metadata is now read into the Sphinx config, dropping the project-specific version discovery invocation. Even its name is loaded from the PEP 621 config rather than being hardcoded. Both are exposed as RST substitutions.

The tox env has been renamed from docs to build-docs and it now prints out instructions for viewing the built docs site locally. The infrastructure includes an optimizationthat speeds up local documentation rebuilds for any documents that render version information.

Finally, the Sphinx invocation command runs in a stricter mode and allows the caller to pass custom arguments at the end, which is relied on in the RTD env but is not significant in the dev envs.

[patchback]

Backport to release_3.1: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply e94e544 on top of patchback/backports/release_3.1/e94e544b9a003040efde05023ea755d01ee302d9/pr-809

Backporting merged PR #809 into devel

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/ansible/ansible-builder.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/release_3.1/e94e544b9a003040efde05023ea755d01ee302d9/pr-809 upstream/release_3.1

4. Now, cherry-pick PR 🧪🤖 Integrate RTD w/ the tox workflow tool natively #809 contents into that branch:

   $ git cherry-pick -x e94e544b9a003040efde05023ea755d01ee302d9

   If it'll yell at you with something like fatal: Commit e94e544b9a003040efde05023ea755d01ee302d9 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x e94e544b9a003040efde05023ea755d01ee302d9

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR 🧪🤖 Integrate RTD w/ the tox workflow tool natively #809 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/release_3.1/e94e544b9a003040efde05023ea755d01ee302d9/pr-809

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to release_3.0: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply e94e544 on top of patchback/backports/release_3.0/e94e544b9a003040efde05023ea755d01ee302d9/pr-809

Backporting merged PR #809 into devel

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/ansible/ansible-builder.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/release_3.0/e94e544b9a003040efde05023ea755d01ee302d9/pr-809 upstream/release_3.0

4. Now, cherry-pick PR 🧪🤖 Integrate RTD w/ the tox workflow tool natively #809 contents into that branch:

   $ git cherry-pick -x e94e544b9a003040efde05023ea755d01ee302d9

   If it'll yell at you with something like fatal: Commit e94e544b9a003040efde05023ea755d01ee302d9 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x e94e544b9a003040efde05023ea755d01ee302d9

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR 🧪🤖 Integrate RTD w/ the tox workflow tool natively #809 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/release_3.0/e94e544b9a003040efde05023ea755d01ee302d9/pr-809

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.