feat: update gatsby to v3 and doctocat

[joshblack]

Note: this PR depends on: primer/doctocat#479

This PR update gatsby to v3 and uses a prerelease version of the doctocat theme which has also been updated to support v3.

Checklist

• Update Gatsby to 3.x
• Update Gatsby-related packages
  • gatsby-plugin-manifest@2.12.1" has incorrect peer dependency "gatsby@^2.25.0".
  • gatsby-plugin-react-helmet@3.10.0" has incorrect peer dependency "gatsby@^2.0.0".
  • gatsby-plugin-styled-components@3.10.0" has unmet peer dependency "babel-plugin-styled-components@>1.5.0".
  • gatsby-plugin-styled-components@3.10.0" has incorrect peer dependency "gatsby@^2.0.32".
  • gatsby-source-filesystem@2.11.1" has incorrect peer dependency "gatsby@^2.2.0".
  • gatsby-transformer-yaml@2.11.0" has incorrect peer dependency "gatsby@^2.0.15".
  • gatsby-plugin-sass@2.8.0" has incorrect peer dependency "gatsby@^2.0.0".
• Update @primer/gatsby-theme-doctocat