doc: Status Page that shows us which SQL functions are supported and which are missing

[CAJan93]

Describe the solution you'd like

A supported_functions.md with the functions we support and that we can update automatically.

We can pull the PSQL functions from the Postgres docs (see e.g. here) and then just check if a function is mentioned in one of our *.slt.* test files.

Think this would be nice to have and it would aid our dev process.

Or to get all PSQL functions:

SELECT
    n.nspname,
    p.proname
FROM 
    pg_catalog.pg_namespace n
JOIN 
    pg_catalog.pg_proc p ON 
    p.pronamespace = n.oid
WHERE 
    p.prokind = 'f'

[xxchan]

Similar idea discussed before #3919
I think the idea is generally OK, but not on the schedule. Are you willing to implement it?

[xiangjinwu]

Updated the tracking page of builtin functions #112