Add docs for new Calendar Editor and validation rules (#249)

* Add docs for new Calendar Editor and validation rules

Expanded "Calendars" docs to cover the January 2026 Tabular Editor 3 Calendar Editor, including usage, layout, calendar management, column mapping, and real-time validation rules. Replaced references to the old column mappings dialog and improved formatting throughout.

* Address PR feedback for Calendar Editor documentation

- Fix typos (Righ-click, duplicate words, Unicode dash)
- Add Calendar > Edit Calendar... menu option
- Fix behavior description (first calendar selected by default)
- Restore Column Mappings Dialog section
- Correct time units table (15 units: 5 complete, 10 partial)
- Fix layout description (left grid, right context panel)
- Update validation rules to match implementation
- Add Calendar Editor screenshots

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Update Calendar Editor docs with improved structure and complete alternatives

- Reorganized documentation to present Calendar Editor before Column Mappings Dialog
- Added SQLBI and Microsoft reference links for calendar-based time intelligence
- Clarified distinction between primary column mapping and associated columns
- Added Time-Related Columns section with lateral/hierarchical shift behavior
- Added complete dependency alternatives for partial time units (Week of Month, Day of Month, Day of Week)
- Added new images for Associated Columns panel, Time-Related Columns panel, and Collection Editor Column Group
- Fixed "Applying Changes" section to clarify local model vs disk save behavior

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Enhance Calendar Editor documentation with step-by-step guidance and screenshots

- Add step-by-step instructions for mapping columns to time units
- Add tooltips documentation for complete and partial time units
- Add example section showing how to use partial time units with data preview
- Add screenshots: dropdown selection, tooltips, dependency errors, cross-calendar validation
- Update partial time units table with bullet points for dependency alternatives
- Add co-author Maria José Ferreira
- Fix validation section descriptions and add validation error images

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Address PR feedback: high-quality images and SQLBI reference link

- Replace all calendar documentation images with higher resolution versions
- Add SQLBI reference link for lateral/hierarchical shift concepts in Time-Related Columns section

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: MariaJoseFF