* provide overriden `hashCode` and toString methods to account for `DISTINCT`
* change the analyzer for scenarios where `COUNT <field_name>` and `COUNT DISTINCT` have different paths
* defined a new `filter` aggregation encapsulating an `exists` query to filter out null or missing values
Enhance error message for the case that the 2nd argument of PERCENTILE
and PERCENTILE_RANK is not a foldable, as it doesn't make sense to have
a dynamic value coming from a field.
Fixes: #36903
* 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
Split function section into multiple chapters
Add String functions
Add (small) section on Conversion/Cast functions
Add missing aggregation functions
Enable documentation testing (was disabled by accident). While at it,
fix failing tests
Improve spec tests to allow multi-line queries (useful for docs)
Add ability to ignore a spec test (name should end with -Ignore)
Andrei Stefan
Marios Trivyzas
Costin Leau