The commit moves legacy.mdx to registration.mdx and significantly
shortens it. Instead of explaining every registration option now a table
is provided. Renaming the page because besides registration there is not
much else to explain about legacy auth. If anything, those should be
elaborated on the main authentication.mdx page (i.e. difference between
OAuth and UIAA)
The email section has also been updated with (some) relevant info.
