* [DOCS][9.0] Improve ESQL reference docs IA
- reorganized es|ql reference documentation from flat list to logical hierarchy
- created three main sections: syntax reference , special fields, advanced operations
- renamed pages with more consistent and task-oriented titles
- aligned navigation titles with page content
- improved introductory text for each section
- used parallel phrasing for similar concepts
- clarified the relationship between reference docs and conceptual docs
Co-authored-by: Alexander Spies <alexander.spies@elastic.co>
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)
