After `TIME` SQL data type is introduced, implement
`CURRENT_TIME/CURTIME` functions similarly to CURRENT_TIMESTAMP
that return the system's current time (only, without the date part).
Closes: #40468
(cherry picked from commit 9feede781409d0e264ce45951a25b28ff129b187)
* Added Limitations page
* Made the aggregations page follow the common template for functions
* Modified all tables to have the first row's cells content centered
* Polishing in other various sections
