Match ecosystem-wide Sphinx doc footer convention - #12

Per rvc-ecosystem AGENTS.md section 5: footer left side reads exactly
"Copyright (c) 20xx-present, Peter Corke: built YYYY-MM-DD", right
side is a smaller-font GitHub link back to the repo. Copies bdsim's
reference implementation (2026-08-16), the pair this convention was
established with:

- docs/source/_templates/footer.html: overrides sphinx_rtd_theme's
  default footer (which reads "(c) Copyright ... Last updated on
  DD-Mon-YYYY") with the exact left-side text, plus the GitHub link.
- docs/source/_static/custom.css: flexbox on the footer's
  contentinfo div so the GitHub link actually renders on the right --
  a plain <a> tag with no CSS just falls inline below the copyright
  line.

Also fixes conf.py to match: copyright was "2020, Peter Corke" (no
"-present"), and html_last_updated_fmt was "%d-%b-%Y" instead of
"%Y-%m-%d" (the format the new footer template actually expects).

As a side effect this also resolves a pre-existing, unrelated build
warning: html_static_path already listed "_static" in conf.py, but
the directory didn't exist on disk, so every build warned "html_
static_path entry '_static' does not exist" -- creating the directory
for custom.css fixes that too.

Verified: real Sphinx build renders the exact expected footer text
("Copyright (c) 2020-present, Peter Corke: built 2026-09-03.") with
the GitHub link correctly positioned via the loaded custom.css.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>