Add basic support for `TOP X` as a synonym to LIMIT X which is used
by [MS-SQL server](https://docs.microsoft.com/en-us/sql/t-sql/queries/top-transact-sql?view=sql-server-ver15),
e.g.:
```
SELECT TOP 5 a, b, c FROM test
```
TOP in SQL server also supports the `PERCENTAGE` and `WITH TIES`
keywords which this implementation doesn't.
Don't allow usage of both TOP and LIMIT in the same query.
Refers to #41195
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
* 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
