Hides some of the "extra" lines from ESQL's documentation. These lines
are required to make the documentation into nice tests which is
important to make sure the docs don't get out of date. But readers don't
need to see them.
In particular:
* Remove all links (both asciidoc and markdown) from the JSON definition files.
* This required a two phase edit, from asciidoc links to markdown, and then removal of markdown (replace with markdown text). This is because the asciidoc does not have the display text, and because some links were already markdown.
* Split predicates into is_null and is_not_null
* We kept the old combined version because the main docs still use that, so now we have both combined and separate versions, and Kibana can select the version they want.
This entirely syncs the contents of docs/reference/query-languages/esql with main, just so we can continue auto-backporting easily, which will continue to be relevant.
* Restructure query-languages docs files for clarity (#124797)
In a few previous PR's we restructured the ES|QL docs to make it possible to generate them dynamically.
This PR just moves a few files around to make the query languages docs easier to work with, and a little more organized like the ES|QL docs.
A bit part of this was setting up redirects to the new locations, so other repo's could correctly link to the elasticsearch docs.
* After running tests, we revert a few features
* Fix build.gradle and improved some generating code
* After running tests
* Update x-pack/plugin/esql/build.gradle
