rename extra.css to custom.css to bust cache

[Roma36]

Description of the change

The recent banner CSS fix revealed the issue we are having with custom css assets. For some reason, MKDocs doesn't provide a out-of-the-box way of cache busting for these assets by appending unique hash string to the file name, for example.

I've also found the open issue: mkdocs/mkdocs#1979

As a hotfix, let's rename extra.css to custom.css and afterwards will think of a long-term solution.

Checklist

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• You have updated the navigation files correctly:
  • No new pages have been added, or;
  • Only nav.yaml has been updated because the changes only apply to SaaS, or;
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  • Both nav.yaml and nav.self-hosted.yaml have been updated.

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[render]

Your Render PR Server URL is https://spacelift-user-documentation-pr-390.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-clirgqd8td7s7385ktc0.