Add site_url to mkdocs.yml (#2326)

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### What

Fix for #2222 (maybe)

It seems like the sitemap for `ref.rerun.io/docs/python/...` is empty.
It can be fixed by adding a `site_url` for it to use as the base url of
links in the sitemap.

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)

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PR Build Summary: https://build.rerun.io/pr/2326

<!-- pr-link-docs:start -->
Docs preview: https://rerun.io/preview/edb65d4/docs
Examples preview: https://rerun.io/preview/edb65d4/examples
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rerun_py/mkdocs.yml

@@ -2,6 +2,7 @@
 # Top-level config for mkdocs
 # See: https://www.mkdocs.org/user-guide/configuration/
 site_name: Rerun Python APIs
+site_url: https://ref.rerun.io/docs/python/latest
 repo_url: https://github.com/rerun-io/rerun/
 
 # Use the material theme
@@ -61,3 +62,4 @@ extra:
   version:
     provider: mike
     default: latest
+