Update site_url for mkdocs

[glennmatthews]

Closes: #n/a

What's Changed

docs.nautobot.com URL has changed. This was mostly updated in #2712 but this one needs to be updated too - the main noticeable symptom is that 404 pages under this project (e.g. https://docs.nautobot.com/projects/core/en/stable/foobar) get rendered incorrectly.

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• Attached Screenshots, Payload Example
• n/a Unit, Integration Tests
• n/a Documentation Updates (when adding/changing features)
• n/a Example Plugin Updates (when adding/changing features)
• n/a Outline Remaining Work, Constraints from Design