Add REP-144 check list to pr template, and split pull request template files

[ijnek]

• Adds REP-144 information and updates the DOC INDEX TEMPLATE
• Since the PR template becomes rather large and readability is reduced, I have also split the pull request templates into mupltiple templates which can be selected from the root pr template.
• Improvements to language and formatting to increase readability of templates.

Supercedes #39987.

[mjcarroll]

Is the PULL_REQUEST_TEMPLATE feature new? I believe that we had tried this in the past without success, you had to put it all in a single markdown file

[ijnek]

As far as I can tell, this feature has been available for a few years. One caveat is that GitHub lacks a user-friendly interface for selecting templates, unlike GitHub issue templates (eg. ros2-gbp's issue template selection).

Without a user-friendly interface, to access the other templates, one needs to utilize a query string in the URL (e.g., ?template=rosdep_rule_template.md). To circumvent this lack of interface, the root PR template is designed to include links to the other specialized PR templates.

Here's a screenshot of the root PR:

[github-actions]

This PR hasn't been activity in 14 days. If you are still are interested in getting it merged please provide an update. Otherwise it will likely be closed by a rosdistro maintainer following our contributing policy. It's been labeled "stale" for visibility to the maintainers. If this label isn't appropriate, you can ask a maintainer to remove the label and add the 'persistent' label.

[ijnek]

Please remove the 'stale' label, this is waiting for review

[github-actions]

This PR hasn't been activity in 14 days. If you are still are interested in getting it merged please provide an update. Otherwise it will likely be closed by a rosdistro maintainer following our contributing policy. It's been labeled "stale" for visibility to the maintainers. If this label isn't appropriate, you can ask a maintainer to remove the label and add the 'persistent' label.