feat: sync common pages across distributions

[AJedancov]

Basic Info

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Ticket(s) this addresses	NA
Does this PR contain AI-generated software?	No

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Description of contribution in a few bullet points

A commit for Jazzy that automatically syncs common pages from the Rolling branch.
To do this, I used the cloning approach that was implemented previously. I've added an extra parameter destination_dir to specify exactly where to clone the files:

github_repositories:
  navigation2:
  ...
  # TODO: Update directory name 'mkdocs.nav2.org' after documentation release
  mkdocs.nav2.org:
    owner: "ros-navigation"
    branch: "rolling" # keep unchanged between distributions
    destination_dir: "docs/shared"
    data_to_clone:
      - "/docs/home"
      - "/docs/community"
      - "/docs/robots_using"
      - "/docs/about_and_contact"

Common subpages, together with their assets, are cloned to docs/shared. Since they're in the main /docs directory, they're automatically included in the documentation build.

As this feature will be used in non-rolling branches, some steps must be repeated each time a new version is released:

1. Completely delete the shared directories (home, community, etc.)
2. Create a new docs/shared directory and add a README file to it, so that there is at least one file and git can track the empty directory.
3. Update .gitignore
4. Exclude the README from build process:

exclude_docs: |
  ...
  shared/README.md

5. Update the paths to common files in the nav configuration
6. Update path to the homepage. It is necessary that clicking the main logo in the header opens the right index page.

I think we can combine steps 3 and 4 and move them to the Rolling branch to simplify the process a little. I've added #37 with these steps for the new documentation release process.

I've also slightly changed the structure in Rolling, now the home index and its images are in a separate directory, so that it's easier to reference it when setting up the macro.

Note1: I intentionally didn't use the .nav.yml file in the new home directory. As this plugin doesn't handle relative links correctly (e.g. "../getting_started/index.md"), and these are needed to create the first two levels of the navigation tree.

Note2: Fetching all the graphic materials from GitHub, particularly the videos for the homepage, increases the time taken for the first build. This is unlikely to be a problem for users, as it only happens once and is then used as a cache, but this will also affect the execution time of each CI pipeline.

[SteveMacenski]

I assume a similar thing will be applied to lyrical

LGTM, just questions on TODOs if they're meant to stay or not

[AJedancov]

I assume a similar thing will be applied to lyrical

Sure, I'll make the same changes there.

I've also removed the homepage from the sync, as mentioned in the main ticket.

[AJedancov]

I see the main concept is approved. Do you mind if I merge this now, even if further discussions/changes will follow? This would simplify the testing of such cross-branch changes.