users-guide: links to extensions will break on ghc-8.12

Summary

There are lots of valuable web documents and pages on nets that refer to the extension pages of the ghc user's guide. The pages are specifying links in the following format:

https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#extension-ApplicativeDo

However, since ghc-8.12, glasgow_exts.html file no longer exists. Therefore, many valuable links are lost. That is, 404 will be displayed as follows:

• https://ghc.gitlab.haskell.org/ghc/doc/users_guide/glasgow_exts.html#extension-ApplicativeDo

Proposed improvements or changes

So we need a redirect mechanism or a rescue page.

One of just idea is to rename users_guide/exts/table.rst to users_guide/glasgow_exts.rst. It's not a perfect solution, but it does prevent a 404.

Any better idea?

Environment

• GHC version used: ghc-8.12