Add sitemap and sitemap meta tag to docs site (#499)

* Initial plan

* Add sitemap meta tag to docs site via MkDocs Material theme override

Co-authored-by: csharpfritz <78577+csharpfritz@users.noreply.github.com>
Agent-Logs-Url: https://github.com/FritzAndFriends/BlazorWebFormsComponents/sessions/0a9bccbb-1c5c-4660-af93-5084ce1fc875

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: csharpfritz <78577+csharpfritz@users.noreply.github.com>

Author: Copilot

docs/_overrides/main.html

@@ -0,0 +1,6 @@
+{% extends "base.html" %}
+
+{% block extrahead %}
+  {{ super() }}
+  <link rel="sitemap" type="application/xml" title="Sitemap" href="{{ config.site_url }}sitemap.xml">
+{% endblock %}

mkdocs.yml

@@ -15,6 +15,7 @@ extra:
 
 theme:
     name: material
+    custom_dir: docs/_overrides
     language: en
     palette:
       scheme: default