* Split ES|QL functions/operators/commands into separate pages for similar functions and make commands examples generated (#126279)
While the internal structure of the docs is already split into many (over 1000) sub-pages, the final display for the `Functions and Operators` page is a single giant page, making navigation harder. This PR splits it into separate pages, one for each group of similar functions and one for the operators. Twelve new pages.
This PR also bundles a few other related changes. In total what is done is:
* Split functions/operators into 12 pages, one for each group, maintaining the existing split of each function/operator into a snippet with dynamically generated examples
* Split esql-commands.md into source-commands.md and processing-commands.md, each of which is split into individual snippets, one for each command
* Each command snippet has it's examples split out into separate files, if they were examples that were dynamically generated in the older asciidoc system
* The examples files are overwritten by the ES|QL unit tests, using a similar mechanism to the examples written for functions and operators)
* Some additional refinements to the Kibana definition and markdown files (nicer operator headings, and display text)
* After re-running tests to ensure we get 9.0 behaviour only
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important to make sure the docs don't get out of date.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
