document: supported database types

[huyennbl]

Please update the documentation about the supported database types. I wander around 15 minutes, still cannot find any information of that.
All I can see is Planned Database Support, but how about Current Database Support?

[kyleconroy]

A language and database support table exists here: https://github.com/kyleconroy/sqlc/blob/main/docs/reference/language-support.rst

It appears that table syntax is wrong because it's not showing up in the docs.