feat(sphinx-autodoc-sphinx): add :exclude: option to autoconfigvalues

why: two extensions (sphinx-gp-sitemap and sphinx-gp-llms) both
register site_url via app.add_config_value(). The autoconfigvalues
directive on both reference pages emits duplicate confval entries,
which Sphinx's RST domain warns about — breaking CI's -W build.

what:
- Add :exclude: option to AutoconfigvaluesDirective that accepts a
  comma/whitespace-separated list of config value names to skip
- Add _parse_exclude() helper with doctests
- Use :exclude: site_url on sphinx-gp-llms reference page
- Document the option in sphinx-autodoc-sphinx tutorial

Author: tony

docs/packages/sphinx-autodoc-sphinx/tutorial.md

@@ -19,3 +19,13 @@ Render every config value from an extension module:
 .. autoconfigvalues:: sphinx_config_demo
 ```
 ````
+
+Exclude specific config values (useful when two extensions register
+the same value and Sphinx warns on the duplicate ``confval``):
+
+````myst
+```{eval-rst}
+.. autoconfigvalues:: sphinx_gp_llms
+   :exclude: site_url
+```
+````

docs/packages/sphinx-gp-llms/reference.md

@@ -9,4 +9,5 @@ Generated from `app.add_config_value()` registrations in
 
 ```{eval-rst}
 .. autoconfigvalues:: sphinx_gp_llms
+   :exclude: site_url
 ```

packages/sphinx-autodoc-sphinx/src/sphinx_autodoc_sphinx/_directives.py

@@ -434,18 +434,40 @@ def run(self) -> list[nodes.Node]:
         )
 
 
+def _parse_exclude(raw: str) -> frozenset[str]:
+    """Parse a comma/whitespace-separated list of config value names.
+
+    Examples
+    --------
+    >>> sorted(_parse_exclude("site_url, html_baseurl"))
+    ['html_baseurl', 'site_url']
+
+    >>> _parse_exclude("")
+    frozenset()
+    """
+    return frozenset(
+        name.strip() for name in raw.replace(",", " ").split() if name.strip()
+    )
+
+
 class AutoconfigvaluesDirective(SphinxDirective):
     """Render all config values registered by one extension module."""
 
     required_arguments = 1
     has_content = False
-    option_spec: t.ClassVar[OptionSpec] = {"no-index": directives.flag}
+    option_spec: t.ClassVar[OptionSpec] = {
+        "no-index": directives.flag,
+        "exclude": directives.unchanged,
+    }
 
     def run(self) -> list[nodes.Node]:
         module_name = self.arguments[0]
         no_index = "no-index" in self.options
+        exclude = _parse_exclude(self.options.get("exclude", ""))
         result: list[nodes.Node] = []
         for value in discover_config_values(module_name):
+            if value.name in exclude:
+                continue
             result.extend(
                 _render_config_value_nodes(self, value, no_index=no_index),
             )