Upgrade antsibull-docs to 2.15.0 and antsibull-core to 3.3.0

[felixfontein]

This should enable collection deprecation information and collection tombstones on the docsites, and fix #1936.

[patchback]

Backport to stable-2.16: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 1abc44e on top of patchback/backports/stable-2.16/1abc44e3a58b092e545cfa0e87125092bf222419/pr-1994

Backporting merged PR #1994 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-documentation.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/stable-2.16/1abc44e3a58b092e545cfa0e87125092bf222419/pr-1994 upstream/stable-2.16

4. Now, cherry-pick PR Upgrade antsibull-docs to 2.15.0 and antsibull-core to 3.3.0 #1994 contents into that branch:

   $ git cherry-pick -x 1abc44e3a58b092e545cfa0e87125092bf222419

   If it'll yell at you with something like fatal: Commit 1abc44e3a58b092e545cfa0e87125092bf222419 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 1abc44e3a58b092e545cfa0e87125092bf222419

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Upgrade antsibull-docs to 2.15.0 and antsibull-core to 3.3.0 #1994 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/stable-2.16/1abc44e3a58b092e545cfa0e87125092bf222419/pr-1994

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 stable-2.17: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 1abc44e on top of patchback/backports/stable-2.17/1abc44e3a58b092e545cfa0e87125092bf222419/pr-1994

Backporting merged PR #1994 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-documentation.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/stable-2.17/1abc44e3a58b092e545cfa0e87125092bf222419/pr-1994 upstream/stable-2.17

4. Now, cherry-pick PR Upgrade antsibull-docs to 2.15.0 and antsibull-core to 3.3.0 #1994 contents into that branch:

   $ git cherry-pick -x 1abc44e3a58b092e545cfa0e87125092bf222419

   If it'll yell at you with something like fatal: Commit 1abc44e3a58b092e545cfa0e87125092bf222419 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 1abc44e3a58b092e545cfa0e87125092bf222419

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Upgrade antsibull-docs to 2.15.0 and antsibull-core to 3.3.0 #1994 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/stable-2.17/1abc44e3a58b092e545cfa0e87125092bf222419/pr-1994

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 stable-2.18: 💚 backport PR created

✅ Backport PR branch: patchback/backports/stable-2.18/1abc44e3a58b092e545cfa0e87125092bf222419/pr-1994

Backported as #1995

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

[felixfontein]

Hmm, what I forgot to add is the update of the docs build scripts in this repo, which need to provide more information to antsibull-docs for some of the new features. It might be better to do that in a follow-up PR though, especially for the stable branches.

[felixfontein]

I created backports to stable-2.17 and stable-2.16 in #2002 and #2003.