docs: add dfinity starlight theme

[nathanosdev]

Description

This PR adds a Starlight plugin with a Dfinity-style theme. It's not perfect (I'm not generally good with colors), but it's much closer to the dev docs now than it was before and this gives a starting point for iterating on that design.

Fixes # (issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

Checklist:

• My changes follow the guidelines in CONTRIBUTING.md.
• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[github-actions]

size-limit report 📦

Path	Size
@dfinity/agent	48.15 KB (0%)
@dfinity/assets	53.21 KB (0%)
@dfinity/auth-client	19.38 KB (0%)
@dfinity/candid	13.22 KB (0%)
@icp-sdk/core	91 B (0%)
@dfinity/identity	18.81 KB (0%)
@dfinity/identity-secp256k1	33.03 KB (0%)
@dfinity/principal	4.44 KB (0%)
@dfinity/use-auth-client	59.09 KB (0%)