Restore /Discord page

[jamesbt365]

We have linked this in numerous places at this point and having this link dead doesn't make a lot of sense We should consider having this automatically redirect to the discord link like the old site.

[gonX]

FWIW this was changed in a7f3cf5
The commit changes more than just disabling the /Discord page (it also changed wiki contents to use a direct link instead), so instead reintroducing redirects (which was removed in 63a247b) and readding the Discord.md file from the first commit, modifying the value to hopefully use the one from _config.yml instead should fix this?