fix(gatsby-link): Adds Type, PropType, tests, and docs for state prop

[ptim]

Description

Adds Type, PropType, tests, and docs for state prop

Documentation

Updates https://www.gatsbyjs.org/docs/location-data-from-props/#example-of-providing-state-to-a-link-component to agree with https://www.gatsbyjs.org/docs/gatsby-link/ and cross links both documents.

Another relevant page is https://www.gatsbyjs.org/docs/linking-between-pages/ but this already links to the GatsbyLink API docs, so I'm avoiding link soup here. Can add more cross linking if required.

Related Issues

Related to: #3552

[blainekasten]

Solid stuff!

Changes were made

[gatsbot]

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[ptim]

Woah - I missed all the action! Thanks so much everyone 😀🙏