feat: Add incomplete Starlight rewrite of docs site

[yhakbar]

Description

Adds an incomplete rewrite of the Terragrunt website rewritten using Starlight.

I'm looking to get this merged into main so that:

1. I can setup a live Vercel preview environment for each pull request.
2. We don't have a massive PR with everything finished at once.

I included a TODO.md with a list of tasks that need to be completed before the rewrite can be considered complete.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added docs-starlight as an incomplete rewrite of the docs site in Starlight.