fix: Ensure site_url is set in the MkDocs config to prevent <loc>None</loc> in sitemap generation

[leocross]

What does this do

Fix a bug with the documentation generating a sitemap with <loc>None</loc> seamingly caused by not having site_url set in the mkdocs config.

e.g.

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <url>
    <loc>None</loc>
    <lastmod>2023-06-15</lastmod>
    <changefreq>daily</changefreq>
  </url>
  ...
</urlset>

Why this is important

Search engines and other tools use the sitemap. Presenting correct information is important for discoverability of the documentation.

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