fix(doc): Internal page-context links to code 404

[duffn]

Description

The links to GitHub in the docs in the internals page.context section 404: https://www.gatsbyjs.org/docs/gatsby-internals-terminology/#pagecontext

This updates the links to point to the current code.

Documentation

This is an update to the docs.

Related Issues

None.

[laurieontech]

This is why having team members in the docs is so awesome. Thanks @duffn!

[gatsbot]

Holy buckets, @duffn — we just merged your PR to Gatsby! 💪💜

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