_redirects in Nix manual is not picked up, leading to broken links

[roberth]

Links such as https://nixos.org/manual/nix/stable/language/constructs.html#functions are currently broken.
The redirect to nix.dev works, but we're serving a 404, despite _redirect being included in our nix#nix-manual output.

• Original report: broken link given by nix build compiler to documentation. nix#13488

[fricklerhandwerk]

This is a general problem that's been known for a while, and a fundamental issue with Netlify redirects:

• Pretty URLs don't strip .html and thus won't get captured by redirect patterns that don't have the suffix
• You can't have *.html in redirect patterns, the * must be at the end of the pattern

That is, https://nixos.org/manual/nix/stable/language/constructs#functions works fine.

We could (programmatically) duplicate and extend all concrete (i.e. non-glob) patterns with .html, that should work.

The alternative, which would require a different type of work, is hosting our sites ourselves instead of through Netlify. I'd personally prefer this, but don't know what's needed to make that happen. @NixOS/infra wdyt?