[20733] Document how to annotate members as key in XML types (backport #746)

[mergify]

Description

This PR documents how to annotate members as key in XML types (@key equivalent)

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

Related implementation PR:

• [20733] Fix support for @key annotation in Dynamic types Fast-DDS#4694

Contributor Checklist

• Commit messages follow the project guidelines.
• Code snippets related to the added documentation have been provided.
• 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 #746 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of f95e5c9 has failed:

On branch mergify/bp/2.6.x/pr-746
Your branch is up to date with 'origin/2.6.x'.

You are currently cherry-picking commit f95e5c9.
  (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:
	modified:   docs/fastdds/xml_configuration/dynamic_types.rst

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   code/XMLTester.xml

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

[Mario-DL]

@richiprosima please test docs