[docs] add Tablegroups to the \h menu

[timothy-e]

Add \h ALTER TABLEGROUP, \h CREATE TABLEGROUP, and \h DROP TABLEGROUP.

Should look like:

yugabyte=# \h ALTER TABLEGROUP
Command:     ALTER TABLEGROUP
Description: change the definition of a tablegroup
Syntax:
ALTER TABLEGROUP name RENAME TO new_name
ALTER TABLEGROUP name OWNER TO { new_owner | CURRENT_USER | SESSION_USER }

There is nothing else about a tablegroup that can be set right now, so nothing else to support altering.

yugabyte=# \h CREATE TABLEGROUP
Command:     CREATE TABLEGROUP
Description: define a new tablegroup
Syntax:
CREATE TABLEGROUP tablegroup_name

No other options are supported currently in the creation of tablegroups.

yugabyte=# \h DROP TABLEGROUP
Command:     DROP TABLEGROUP
Description: remove a tablegroup
Syntax:
DROP TABLEGROUP name

Other DROP statements support IF EXISTS (e.g. DROP TABLESPACE IF EXISTS tblspace) but tablegroups do not.

[polarweasel]

@timothy-e this isn't actually a docs issue, since the \h output comes from the YSQL executable itself. Please ping @m-iancu to assign this one to the correct team member(s).