docs: move RELEASING.md out of the public repo

The release process is a maintainer-internal recipe — there's no
external audience for the publish-RC-then-publish-real flow, the
TestPyPI/PyPI configuration, or the workflow gating table. Moving
the file out of the public repo prevents an external reader from
mistaking it for guidance they should follow.

Changes:

- ``docs/RELEASING.md`` removed from this repo. The full content is
  now in the private ``openarmature-coord`` repo under
  ``docs/openarmature-python-RELEASING.md``.
- ``mkdocs.yml`` ``exclude_docs: RELEASING.md`` removed — the file
  no longer needs an exclusion since it no longer exists here.
- ``docs/contributing/index.md`` no longer links to RELEASING.md;
  the page becomes a stub.

Past CHANGELOG entries reference ``docs/RELEASING.md`` (e.g., the
0.5.0 entry). Those are snapshot history and intentionally not
edited — the references describe what was true at that release.

Author: chris-colinsky

docs/RELEASING.md

@@ -1,7 +1,5 @@
 # Contributing
 
-Contributor-facing docs (development setup, release process, etc.)
-land here. For the current release process, see
-[`docs/RELEASING.md`](https://github.com/LunarCommand/openarmature-python/blob/main/docs/RELEASING.md)
-in the repo (the file is contributor-internal and intentionally not
-rendered as a docs page).
+Contributor-facing docs (development setup, repo conventions, etc.)
+land here. Stub for now — populated as concrete contributor-facing
+material accrues.

docs/contributing/index.md

@@ -10,12 +10,6 @@ edit_uri: edit/main/docs/
 # on the maintainer's workstation; 8765 is the project convention.
 dev_addr: "127.0.0.1:8765"
 
-# RELEASING.md is contributor-facing — not part of the rendered site.
-# (A future PR may fold it into ``contributing/`` as a proper page.)
-exclude_docs: |
-  RELEASING.md
-
-
 theme:
   name: material
   palette: