Docs: add sign up and log in CTA to docs footer
Add a call-to-action banner in the docs footer with sign up and log in
buttons that open modals for choosing between Community and Business
platforms, matching the website pattern.
Also fix templates_path in conf.py to use an absolute path so the
multiproject plugin srcdir override does not break template resolution.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
