Standardize maturity field across all collection YAML manifests

Summary

An audit of all 14 collection YAML manifests against the Docusaurus site data (docs/docusaurus/src/data/collectionCards.ts) reveals inconsistent, missing, and conflicting maturity metadata. This blocks reliable downstream consumption by #1317 (dynamic Docusaurus generation) and #1318 (maturity badge visuals), and undermines the maturity filtering added in #1256.

/cc @katriendg

Current State

Key Findings

1. Only 3 of 14 manifests have a collection-level maturity: field (experimental, design-thinking, rai-planning)
2. Zero manifests use the value stable — the most common Docusaurus assignment
3. 3 direct conflicts where Docusaurus assigns Stable but plugin-level YAML says experimental (coding-standards, data-science, project-planning)
4. Case mismatch: YAML uses lowercase (experimental, preview) while Docusaurus uses title-case (Experimental, Stable, Preview) — normalization needed at consumption boundaries
5. Placement inconsistency: Existing maturity: fields appear at line 4 in some files and line 9 in others
6. 2 meta-collections (hve-core-all, installer) have no Docusaurus presence and no maturity data — their maturity level needs a product decision

Impact

• Dynamically generate Docusaurus collection pages from YAML manifests #1317 (dynamic Docusaurus generation) requires reliable collection-level maturity to render badges and filter views
• Enhance maturity badges with icons and extend to collection detail pages #1318 (maturity badge visuals) depends on consistent maturity values across the pipeline
• Prepare-Extension.ps1 maturity filtering (from collection.md: auto-generation, maturity safety, and custom header support #1256) operates on collection-level maturity: — only 3 of 14 collections are currently filterable
• Validate-Collections.ps1 does not enforce presence of maturity: — missing values pass validation silently

Proposed Remediation

1. Add maturity: to all 14 manifests

Place the field consistently after description: (line 4) using lowercase values (stable, preview, experimental). Reference values from Docusaurus as the source of truth for the 12 collections it covers.

Proposed assignments:

2. Add validation rule

Update Validate-Collections.ps1 to enforce that every collection YAML manifest includes a maturity: field with an allowed value (stable, preview, experimental).

3. Resolve the conflict question

Decide whether the 3 conflicting collections (coding-standards, data-science, project-planning) should be stable (matching Docusaurus) or experimental (matching their plugin-level items). This may require a product conversation about whether collection maturity represents the collection packaging or the content maturity.

4. Determine meta-collection maturity

Decide the maturity level for hve-core-all and installer, which are not represented in Docusaurus. Suggested default: stable since they are packaging meta-collections wrapping stable content.

Acceptance Criteria

• All 14 collection YAML manifests contain a maturity: field
• maturity: is placed consistently after description: across all manifests
• Values use lowercase: stable, preview, or experimental
• Validate-Collections.ps1 enforces maturity: presence and allowed values
• Conflicts between Docusaurus and YAML are resolved with documented reasoning
• Meta-collection maturity values are decided and documented
• Plugin generation (npm run plugin:generate) succeeds after changes
• All existing tests pass (npm run test:ps)
• Lint passes (npm run lint:all)