Use sphinx.ext.extlinks for GitHub issue and PR links.

Added `sphinx.ext.extlinks` to the Sphinx configuration and defined `extlinks` for `:issue:` and `:pr:` roles, pointing to the MuJoCo GitHub repository. Updated `changelog.rst` to use these new roles, replacing `:github:issue:` and `:github:pull:` which are currently failing.

Also fix a formatting bug in the 3.3.7 changelog

PiperOrigin-RevId: 828403002
Change-Id: I7f0134e4dd555fc59a3fee090338de56aababe1c

doc/conf.py

@@ -46,6 +46,7 @@ extensions = [
     'sphinx.ext.autodoc',
     'sphinx.ext.napoleon',
     'sphinx.ext.viewcode',
+    'sphinx.ext.extlinks',
     'sphinxcontrib.bibtex',
     'sphinxcontrib.katex',
     'sphinxcontrib.youtube',
@@ -58,6 +59,15 @@ extensions = [
     'mujoco_include',
 ]
 
+# Links to GitHub issues and pull requests.
+extlinks = {
+    'issue': (
+        'https://github.com/google-deepmind/mujoco/issues/%s',
+        'issue #%s',
+    ),
+    'pr': ('https://github.com/google-deepmind/mujoco/pull/%s', 'PR #%s'),
+}
+
 # MuJoCo Warp documentation
 napoleon_google_docstring = True
 autodoc_class_signature = 'separated'