Restore docs.json user guide structure from PR #16705 (#16741)

## Summary
The merge conflict resolution from PR #16705 incorrectly discarded the
new documentation structure changes. This PR restores the intended
changes.

## Changes restored
- New 'Capabilities' and 'How-Tos' subgroups organization
- Renamed sections (e.g., 'Getting Started' → 'Discover Twenty')
- New sections: Data Migration, Calendar & Emails, AI, Views &
Pipelines, Dashboards, Permissions & Access, Billing
- Reorganized Developers section with API subsection
- URL redirects for SEO and user experience continuity

## Context
PR #16705 was merged but the merge conflict was incorrectly resolved,
causing all the structural changes to be lost. This PR brings back those
changes from the final commit of PR #16705
(`c856e0d598a0056c2bdaf528502e08261daf7c7c`).

Author: FelixMalfait