docs: Fix HTML build with Sphinx 8.2

Sphinx 8.2 changed the definition of `depart_admonition`, causing build
failures when building with html-docs=enabled as mesa was only
overloading `visit_admonition`. This adds the old `depart_admonition`
definition to mesa's copy, restoring compatibility and fixing the build.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12725
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34165>

docs/_exts/bootstrap.py

@@ -71,6 +71,9 @@ class BootstrapHTML5TranslatorMixin:
             self.body.append(str(admonitionlabels[name]))
             self.body.append('</div>')
 
+    def depart_admonition(self, node) -> None:
+        self.body.append('</div>\n')
+
     def visit_table(self, node):
         # init the attributes
         atts = {}