[21256] Migrate Fast DDS Docs CI to Github (backport #831)

[mergify]

Description

This PR includes GitHub CI of this repo.
In this case, only Ubuntu workflow that builds Fast DDS docs, taking into account the Fast DDS and Fast DDS python branches.

Note: While backporting this PR, please ensure the following changes:

• Remove nightly jobs
• Update ubuntu-ci.yml file with the proper fallback branches
• Change fastdds.repos to fastrtps.repos
• 2.6.x requires building on ubuntu-20.04

@Mergifyio backport 2.14.x 2.13.x 2.10.x 2.6.x

Contributor Checklist

• Commit messages follow the project guidelines.
• N/A Code snippets related to the added documentation have been provided.
• N/A Documentation tests pass locally.
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• CI passes without warnings or errors.

---

This is an automatic backport of pull request #831 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2266b1a has failed:

On branch mergify/bp/2.6.x/pr-831
Your branch is up to date with 'origin/2.6.x'.

You are currently cherry-picking commit 2266b1a.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   .github/workflows/config/build.meta
	new file:   .github/workflows/config/test.meta
	new file:   .github/workflows/config/test.repos
	new file:   .github/workflows/nightly-ubuntu-ci.yml
	new file:   .github/workflows/reusable-ubuntu-ci.yml
	new file:   .github/workflows/ubuntu-ci.yml
	new file:   RELEASE_SUPPORT.md

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/notes/versions.rst

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[JesusPoderoso]

LGTM with green CI