[Issue #26] Update website content

[widal001]

Summary

Updates website content and information architecture.

• Fixes [Website] Update content and IA #26
• Time to review: 10 minutes

Changes proposed

What was added, updated, or removed in this PR.

• Updates the information architecture for the site
• Removes stale content from Starlight template
• Adds placeholder guides and reference docs
• Updates the site to use the new "CommonGrants" working title
• Improves link checking
• Adds support for "Edit page" link

Context for reviewers

Testing instructions, background context, more in-depth details of the implementation, and anything else you'd like to call out or ask reviewers. Explain how the changes were verified.

1. Check out the PR
2. Change directory into the website/ sub-directory
3. Run npm run dev
4. Go to http://localhost:4321/simpler-grants-protocol/

Additional information

Screenshots, GIF demos, code examples or output to help show the changes working as expected.