Proxito 404: serve custom 404 page from default version on external/PR domains

[stsewd]

What's the problem this feature will solve?

When serving external versions, we force all calls to storage to be to /external/html/... by setting the self.version_type attribute at the start of the request

readthedocs.org/readthedocs/proxito/views/serve.py

Lines 383 to 384 in b13a321

	if unresolved_domain.is_from_external_domain:
	self.version_type = EXTERNAL

readthedocs.org/readthedocs/proxito/views/serve.py

Lines 583 to 588 in b13a321

	storage_root_path = project.get_storage_path(
	type_="html",
	version_slug=version_404.slug,
	include_file=False,
	version_type=self.version_type,
	)

Which is good, but when handling 404s, we try to serve the 404.html file from the default version, which isn't external, and obviously it doesn't exist in the /external/ path.

Describe the solution you'd like

We need to decide what do to in this case:

• Skip serving custom 404s from the default version when we are on an external domain.
• Allow serving the content from the default version, if so, we need to remove the dependency on self.version_type Proxito: remove version_type #10220.

Additional context

#10220

[humitos]

Skip serving custom 404s from the default version when we are on an external domain.

I think this "solution" is fine for now. I don't think we need to complicate this too much.

I find a little weird the case where the "PR doesn't have a 404.html but the default version does". The other way around seems more common, but that's tracked/solved in #10690, so 👍🏼

I wouldn't do anything at this point for this particular issue.