Add docs/developer/editing_docs.md documenting the MyST/Sphinx conventions
for this site: page anatomy (cross-reference label + single H1), language-
tagged code fences, admonitions, cross-references, the {download} role for
non-image files, the {figure} directive, and how to add a page to a toctree.
Also covers local preview, the merge-then-RTD-build flow, and pitfalls
(broken refs are warnings, public visibility, placeholder consistency).
Turn the empty developer.md stub into a landing page with a toctree and a
card linking to the new guide.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
menzel