feat(theme): update gatsby to v3

[joshblack]

This PR is a proof-of-concept for updating the minimum Gatsby version for @primer/gatsby-theme-doctocat to v3 (I figured it would be easier to coordinate moving from v2 to v3 first and then v4 afterwards)

To get there, this PR does the following:

• Update gatsby to v3
• Raise the peerDependency range of gatsby to 3.x
• Update gatsby-plugin-* packages to the range which supports gatsby@v3
• Update gatsby-* packages to the range which supports gatsby@v3

It also updates the docs folder to use plugins instead of __experimentalThemes

Overview

• Gatsby updated to v3
• Remove gatsby-source-filesystem default to content
• Catch links is now built-in to Gatsby
• React LSV is now v17

[changeset-bot]

🦋 Changeset detected

Latest commit: 3b3d8f6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/gatsby-theme-doctocat	Major

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[colebemis]

Thanks for doing this, @joshblack!

Let's test the canary release published from this PR in our documentation sites before merging this

[joshblack]

Closing in favor of #479 which attempts the react migration separately