[BUILD] Check for tabs and nocommits in the code on validate

This commit adds checks for nocommit and tabs in the source code.
The task is executed during the validate phase and can be disabled via
`-Dvalidate.skip`
[BUILD] Generate source jars for tests

Closes #6125
[DOCS] Fixed typo in post-filter.asciidoc

Remove 'be' where it is not needed
[BUILD] exclude target dir from tab validation
Update nested-query.asciidoc

Added note that fields inside a nested query must be full qualified.
Docs: Updated CONTRIBUTING.md to provide more details about the PR process

Closes #6323