Build a specific table showing missing MySQL functions

[breezewish]

Currently we provide a link to the grammar graph, which is not friendly enough. Users still don't have an idea of what functions are missing.

[morgo]

I have added 'unsupported' for specific areas in docs:
pingcap/docs#657
pingcap/docs#656

[morgo]

This is now pingcap/docs#672

[QueenyJin]

Closing this since this is already tracked in the docs repo.