[21777] Solve the issue of the APIs not being generated

[juanjo4936]

Description

Modified conf.py so that the default branch if not set by environment is 1.1.x for the python repository and 2.6.x for the FastDDS repository (both were wrongly set to master). Modified the surrounding code to present the structure in 2.10.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.
• N/A 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.