Exact redirects: allow use of $lang and $version

[DanielNoord]

Details

• Read the Docs project URL: https://pylint.pycqa.org/en/latest/
• Read the Docs username (if applicable): danielnoord

Expected Result

We're a moving a folder in our documentation. Specifically the move is:
doc/messages/* to /doc/user_guide/messages/*.

After messing around with the redirects (a lot) we finally figured out how to set up a global exact redirect for all pages that fell under the messages directory (over 250+ pages, a page for each message pylint can emit).
One of the issues we ran into was that we couldn't use the $lang or $version variables. So, for the relevant PR (pylint-dev/pylint#6628) we had to do:

Type: Exact Redirect
From URL: /en/6628/messages/$rest
To URL: /en/6628/user_guide/messages/

This allowed us to test the PR, but then after the final merge we would need to add this as well:

Type: Exact Redirect
From URL: /en/latest/messages/$rest
To URL: /en/latest/user_guide/messages/

While this works for now, if we bump our version from 2.14 (current) to 2.15 the 2.14 documentation would break unless we add a redirect for /en/2.14/.
Similarly, all other languages won't benefit from this redirect as we only capture en.

Although I understand the concern that by using $version you would also redirect (for example) /en/2.5/ which doesn't have the directory moved, this shouldn't be an issue. Since the move hasn't taken place /en/2.5/messages/* shouldn't return any 404's and therefore no redirect will occur.

As far as I can see the only issue would be /en/2.5/messages/non-exisiting-page.html > /en/2.5/user_guide/messages/non-existing-page.html. But the effect of that would be the same: a 404. The change in url for that 404 is a risk/bug I would be willing to take.

Actual Result

Allow use of $lang and $version in Exact Redirects.

If the proposal is accepted and somebody could point me to the code that is currently handling these redirects I would be willing to take a look myself. I have no idea of how this is coded, but I feel like copying some of the expansion that happens for Page Redirects could be copied to Exact Redirects to fix this?

[humitos]

While this works for now, if we bump our version from 2.14 (current) to 2.15 the 2.14 documentation would break unless we add a redirect for /en/2.14/.

We've seen that Docs.rs handles this properly with these types of redirects: https://docs.rs/about/redirections

It may be a good idea to implement them in Read the Docs.

[stsewd]

This could also be solved by adding the "$rest keyword" feature from exact redirects to page redirects.

[stsewd]

This will be fixed with #10881. We decided to solve this use case by allowing wildcards in page redirects.