[Docs] Add change to enable 404 page automation

[stichbury]

Description

Small change to follow instructions in RTD guide and enable a 404 page. Setting the site_url to the base canonical URL for the docs is sufficient for the tool to build us a 404.html, which we need now I've moved some pages.

Screenshot

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[stichbury]

Just a clarifying question - after this PR, a link like this: https://vizro.readthedocs.io/en/stable/pages/user_guides/custom_actions/#interacting-with-dashboard-inputs-and-outputs

should now lead to a 404 page right? Because it seems like this section doesn't exist anymore? Ah no wait, this seems to exist, but the forward reference doesn't seem to work with this one?

The links to sections within a page should not break a link if the section is renamed or removed. From my testing, if the page exists, it'll be displayed but when the subsection isn't found, it just falls back to show the top of the page.

It sounds like that's another redirect issue, which seems to be ongoing today, but that page may not be in my top 20 list. Maybe I'll just add them all for safety!