Upgrade Process-PSModule automation to v6.1.19

[MariusStorhaug]

Context and request

Maintain the module's automation on the current PSModule/Process-PSModule release so builds, tests, publication, and documentation follow the supported pipeline contract.

Acceptance criteria

• .github/workflows/Process-PSModule.yml pins the reusable workflow to the immutable commit for v6.1.19.
• The caller continues to map only the required APIKey secret explicitly.
• Documentation builds use the Zensical configuration expected by Process-PSModule v6.1.13 and later; the obsolete MkDocs configuration is removed.
• The migrated configuration follows the current PSModule/Template-PSModule convention and retains the module documentation site's name, repository links, icons, navigation, search, markdown extensions, and social links.
• YAML and TOML configuration validation succeeds.

Constraints and non-goals

• Pin the reusable workflow to its release commit SHA rather than a mutable tag.
• Do not change module source, public behavior, or publishing credentials.
• Incorporate Bump actions/checkout from 7.0.0 to 7.0.1 #251 into the delivery pull request after its draft is created, as requested.

Technical decisions

• Update .github/workflows/Process-PSModule.yml to bf67cd90269ca5ce25cd76b203678907dc2984b4 (v6.1.19).
• Preserve the existing APIKey: ${{ secrets.APIKey }} mapping; the workflow contract requires it and GitHub secret names are case-insensitive.
• Replace .github/mkdocs.yml with .github/zensical.toml based on the current PSModule/Template-PSModule configuration, preserving the repository's templated values.
• Validate the changed YAML with yamllint and parse the TOML configuration using PowerShell before opening the draft pull request.

Implementation plan

• Verify the v6.1.19 caller contract and the repository's current configuration.
• Validate the existing YAML and prepare the Zensical configuration migration.
• Update the workflow pin and replace the legacy documentation configuration.
• Validate the changed YAML and TOML configuration.
• Create and push a micro commit, then open a draft delivery pull request.
• Merge Bump actions/checkout from 7.0.0 to 7.0.1 #251 into the draft pull request and update its validation evidence.

[MariusStorhaug]

Updated the implementation plan after delivery: the v6.1.19 caller upgrade and Zensical migration are complete, draft PR #270 is open, and PR #251 was merged into its branch. Workflow YAML, TOML, and both immutable action pins were validated. The task remains open until the draft PR completes review and merges.