feat: migrate documentation to Starlight

[huijing]

Changes proposed in this pull request

This PR sets up a new Starlight documentation site within the repository, and moves our documentation off Readme.io onto Github pages (most likely, hosting solution TBD)

Context

Closes #287

To run this locally:

1. git clone https://github.com/interledger/open-payments.git
2. cd open-payments
3. git checkout chj/287/migrate-docs-to-starlight
4. pnpm i
5. cd docs
6. pnpm dev

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