For the first one, the correct link is:
https://m-labs.hk/migen/manual/ (with a slash at the end)
The corresponding nginx syntax is:

location /migen/manual/ {
  alias /var/www/m-labs.hk.old/migen/manual/;
}

Do you know the correct nginx configuration for accepting both URLs and without this issue? I couldn't figure it out and got frustrated with it.

ragnite While you are updating the page, I recommend adding this link:
Litex Migen Extension

Many of the features presented on this page are not from LiteX but from MiSoC (or even Milkymist); however there is not a single mention of MiSoC on the whole page. I will link to it after credit is properly attributed.

The old tutorial link is fixed (but with a different URL).