Bump @primer/gatsby-theme-doctocat from 3.1.1 to 4.0.0

Bump Doctocat theme to v4 for a consistent reader experience when navigating between different implementation doc sites.

• Bump to PRC v35 and Doctocat v4
• Remove deprecated Grid, ButtonOutline, Flex and BorderBox components. Use Box component instead
• Adjust old LabelGroup and Label to new variant/styles as a temporary solution for the Rails/Figma/React links in the header. We might need to include these links inside the Layout component in Doctocat.
• Adjust the Heading component to use h4 tags

Reference

• https://github.com/github/primer/issues/1192
• https://github.com/primer/design/pull/266