Enable PDF and HTML downloads on ReadTheDocs

[YadavAkhileshh]

Summary

This PR enables downloadable documentation formats (PDF, HTML zip, ePub) on ReadTheDocs.

Fixes #470

Problem

Users cannot download MuJoCo documentation for offline use. PDF builds were previously disabled due to build failures (see TODO comment in .readthedocs.yml line 6).

Solution

• Enabled formats in .readthedocs.yml: pdf, htmlzip, epub
• Configured xelatex engine in doc/conf.py for proper Unicode/math symbol handling
• Set latex_theme to manual for PDF compatibility (Furo theme is HTML-only)

Changes

• .readthedocs.yml: Uncommented and configured formats section
• doc/conf.py: Added LaTeX/PDF build configuration

Testing

• Verified LaTeX build generates files successfully
• Configuration syntax validated locally
• ReadTheDocs will build and verify PDF generation

Benefits

• Users can download documentation as PDF for offline reading
• Better accessibility with multiple download formats
• Matches standard ReadTheDocs documentation features

[thowell]

@YadavAkhileshh thank you for this contribution to mujoco!

i think we should be to verify that downloadable formats are available by checking the build for this pr builds/30813332

• do we know where the links for pdf/htmlzip/epub are located?
• are all of the changes to conftest.py necessary?

thanks!