docs(developer): add "How to edit these docs" contributor guide

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>

docs/developer/developer.md

@@ -1,3 +1,33 @@
 (developer)=
-# Developer 
+# Developer
+Information for developers and maintainers of the cSAXS beamline documentation and software.
 
+```{toctree}
+---
+maxdepth: 2
+hidden: true
+---
+
+editing_docs
+
+```
+
+
+***
+
+````{grid} 2
+:gutter: 5
+
+```{grid-item-card}
+:link: developer.editing_docs
+:link-type: ref
+:img-top: /assets/index_contribute.svg
+:text-align: center
+:class-item: index-card
+
+## Editing the documentation
+
+Conventions for writing these MyST/Sphinx docs and how changes go live on Read the Docs.
+```
+
+````