Add Sphinx domains for intersphinx consumers

[JeanChristopheMorinPerso]

I introduced two new Sphinx domains in 3.4.0 without realizing that it's impossible to reference non-standard domains in other sphinx projects.

This PR creates a new Sphinx extension that our users can use to more easily reference our new domains using intersphinx.

Amp-Thread-ID: https://ampcode.com/threads/T-019fdefd-f7a9-75c8-8d27-ddb0ca62d6fe

[codecov]

Codecov Report

❌ Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.25%. Comparing base (5c598c5) to head (0dd9b19).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/utils/sphinxext.py	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2183      +/-   ##
==========================================
- Coverage   61.29%   61.25%   -0.04%     
==========================================
  Files         164      165       +1     
  Lines       20568    20581      +13     
  Branches     3575     3575              
==========================================
  Hits        12607    12607              
- Misses       7089     7102      +13     
  Partials      872      872              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

This PR adds a lightweight, installable Sphinx extension (rez.utils.docs) that registers Rez’s custom Sphinx domains so external documentation projects can resolve references to Rez’s non-standard domains via intersphinx.

Changes:

• Added rez.utils.docs Sphinx extension that registers the rex and pkgdef domains (PythonDomain-derived).
• Updated Rez’s internal docs extension (docs/rez_sphinxext.py) to reuse the new domain definitions.
• Added a new user guide describing how to configure intersphinx and author external references, and linked it into the guides index.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/rez/utils/docs.py	New public Sphinx extension module that defines and registers lightweight rex/pkgdef domains for intersphinx consumers.
docs/source/guides/linking_to_rez_documentation.rst	New guide explaining how to configure intersphinx + the Rez extension and how to write external references.
docs/source/guides/index.rst	Adds the new “linking_to_rez_documentation” guide to the guides toctree.
docs/rez_sphinxext.py	Refactors Rez’s docs-only extension to subclass/reuse the new domain definitions.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.