build: add docs to github.in & ensure ArbiBOM sees changes to it (#31824)

Nine different URL-specified Python dependencies had
accumulated in edx-platform, noticably increasing the amount
time it took to install edx-platform requirements.

After many developer hours of work, we are nearly finished
converting them to PyPI-hosted depencencies:
openedx/wg-developer-experience#153

We're making these updates in hopes that we don't end
up with so many URL-specified depencencies again.
We're also updating OEP-18:
https://github.com/openedx/open-edx-proposals/pull/450

The CODEOWNERS update would mean that Arbi-BOM is notified
on all future PRs that change github.in. At the moment, edx-platform
does not *require* CODEOWNER review, so this is not a *hard* gate to
github.in changes.

.github/CODEOWNERS

@@ -1,5 +1,9 @@
 # This does not cover all the code in edx-platform but it's a good start.
 
+# Ensure that the team responsible for upgrades sees any PRs that would
+# add GitHub-hosted dependencies to that platform.
+requirements/edx/github.in                                          @openedx/arbi-bom
+
 # Core
 common/djangoapps/student/
 common/djangoapps/student/models/__init__.py                        @openedx/teaching-and-learning