Changes

• Adds seed script documentation, creating (or updating) CMS records for all docs/*.md files.
• Adds a documentation page (if none exists; with slug documentation) and uses it as a parent page.
• Adds a page partial (if none exists; titled "Documentation index") as a convenient list of links to all documentation pages.
• Note: dependencies are kept outside root package.json to resolve conflicts. Also the seed dependencies are not needed for other projects using Head Start.

Associated issue

Related to #27, though that issue is mainly about seeding required content.

How to test

1. Check out this feature branch
2. Run npm run seed:docs (you may want to switch to a new sandbox environment for this)
3. Verify the docs are added to the current DatoCMS environment (in ./datocms-environment.ts)
4. Update a documentation file
5. Re-run the script
6. Verify the related record in the CMS is updated
7. Review the documentation pages in the browser
8. Verify links in the pages (relative and external) work correctly

Checklist

• I have performed a self-review of my own code
• I have made sure that my PR is easy to review (not too big, includes comments)
• I have made updated relevant documentation files (in project README, docs/, etc)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• I have notified a reviewer