Remove legacy docsite

[camertron]

What are you trying to accomplish?

This PR removes our legacy Gatsby docsite and all related CI config, rake tasks, inline examples, etc.

Integration

No prod changes necessary.

List the issues that this change affects.

Partially addresses https://github.com/github/primer/issues/2014
Closes https://github.com/github/primer/issues/1629

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

Accessibility

• No new axe scan violation - This change does not introduce any new axe scan violations.

[changeset-bot]

⚠️ No Changeset found

Latest commit: c12a9b0

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[jonrohan]

This is exciting 💯

[jonrohan]

Seems good 👍🏻

[keithamus]

🔥