* [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>
