Show some useful links on 404 default page

[humitos]

NOTE: this is a POC to discuss what we want to show on this page. Do not merge yet.

With these changes, I'm adding

• default site documentation (based on default version and language)
• the root of the current version and language
• the current page requested that produces the 404

I'd like to add some style to it and show a message similar to

The page /en/auto-wipe/notfound you requested does not exist. You can access the full documentation at {{ default site docs }}.

This way, lost readers will be able to "stay on the project's docs" instead of need to manually manipulate the URL to remove some parts.

Closes #2551

[ericholscher]

Useful approach. Definitely needs some styling love :)

[ericholscher]

Does this suffer from the Django security issue they just patched? django/django@1cd00fc

[stsewd]

We are already with the patched version https://github.com/rtfd/readthedocs.org/blob/ee2d7847240d05c4b7e494ec68b3fc2a654e8631/requirements/pip.txt#L18

[ericholscher]

Right, but we aren't calling urlquote, which is the fix for the path issue.

[humitos]

Closing this PR in favor of #5130. With that PR merged, the user will be able to write the 404 page by themselves and even use a sphinx extension to easily generate it.

Also, we are thinking on including the extension in the common build process and default a generic message. By doing this, each version will have its own 404 and the menu will link to the proper sections of that version where we are seeing the 404 page.