feat: fix junction toggle persistence and add type-safe documentation paths (#17421)

## Summary

- **Fix junction relation toggle not being saved**: The form schema
wasn't tracking the `settings` field, so changes to
`junctionTargetFieldId` weren't marked as dirty
- **Add type-safe documentation paths**: Generate TypeScript constants
from `base-structure.json` to prevent broken documentation links
- **Create many-to-many relations documentation**: Step-by-step guide
for building many-to-many relations using junction objects
- **Update `getDocumentationUrl`**: Now uses shared constants from
`twenty-shared` for base URL, default path, and supported languages

## Key Changes

### Junction Toggle Fix
- Added `settings` field to the form schema in
`SettingsDataModelFieldRelationForm.tsx`
- Fixed the toggle to properly merge settings when updating
`junctionTargetFieldId`

### Type-Safe Documentation Paths
- New constants in `twenty-shared/constants`:
- `DOCUMENTATION_PATHS` - All 161 documentation paths as typed constants
  - `DOCUMENTATION_SUPPORTED_LANGUAGES` - 14 supported languages
  - `DOCUMENTATION_BASE_URL` / `DOCUMENTATION_DEFAULT_PATH`
- Generator script: `yarn docs:generate-paths`
- CI integration: Added to `docs-i18n-pull.yaml` workflow

### Documentation
- New article:
`/user-guide/data-model/how-tos/create-many-to-many-relations`
- Updated `/user-guide/data-model/capabilities/relation-fields.mdx` with
Lab warning and link

## Test plan
- [ ] Verify junction toggle saves correctly when enabled/disabled
- [ ] Verify documentation link opens correct localized page
- [ ] Verify `yarn docs:generate-paths` regenerates paths correctly

packages/twenty-docs/user-guide/data-model/capabilities/relation-fields.mdx

@@ -37,12 +37,16 @@ Many records in Object A can be linked to many records in Object B.
 
 **Example:** Many People can be linked to many Projects, and vice versa.
 
-<Warning>
-**Many-to-Many is not yet supported.**
+Many-to-many relations use a **junction object** pattern: an intermediate object that connects both sides. With the junction relation feature, Twenty displays the final linked records directly, hiding the intermediate object from the UI.
 
-This relation type is planned for H1 2026. As a workaround, create an intermediate "junction" object (e.g., "Project Assignments") that has Many-to-One relations to both objects.
+<img src="/images/user-guide/fields/junction-relation-diagram.png" style={{width:'100%'}}/>
+
+<Warning>
+**Lab Feature**: Junction relations must be enabled at **Settings → Updates → Lab** before use.
 </Warning>
 
+See [How to Create Many-to-Many Relations](/user-guide/data-model/how-tos/create-many-to-many-relations) for a complete step-by-step guide.
+
 ## Creating a Relation Field
 
 1. Go to **Settings → Data Model**