Docs: add sign up and log in CTA to footer

[ericholscher]

Summary

• Adds a call-to-action banner to the docs footer with Sign Up and Log In buttons
• Buttons open modals for choosing between Community (readthedocs.org) and Business (readthedocs.com), matching the website pattern
• Fixes templates_path in conf.py to use an absolute path so the multiproject plugin's srcdir override doesn't break template resolution

Test plan

• Build docs locally (PROJECT=user make html) and verify the CTA appears at the bottom of every page
• Click Sign Up button — modal should show Community vs Business options
• Click Log In button — modal should show Community vs Business login links
• Test modal close via X button, overlay click, and Escape key
• Check responsive layout on narrow viewports (cards should stack)

🤖 Generated with Claude Code

[ericholscher]

I've been wanting this for a long time, so figured it would be a good hack for AI. If we have any feedback, a prompt to give back to the AI to fix it would definitely be useful, since this is all AI-gen. 💯