Check redirects on CI

[verytactical]

Check that every page that was previously avaiable by URL is either still available, or there is a redirect.

If there is a page that was completely removed, and no possible target where it should land, redirect should land on 404 or main page.

[novusnota]

There are actually two kinds of redirects:

1. URLs in the previous TON Docs repository — those have to be reflected in the current structure or the redirects array
2. URLs in the upstream version of the current structure must either coincide with URLs in the local PR docs.json structure or have an entry in the redirects array of the latter

I'm working on making a CI for both.

UPD: And there should also be a check for the validity of the redirects array since Mintlify only checks it upon the CLI start.