To avoid having to specify each spec by hand (which can miss specs to be
added), the test infrastructure now performs classpath discovery so that
each spec added, is automatically considered.
Relates #40358
* 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)
