Update references to the main branch
Conflicts:
.github/CONTRIBUTING.md
website/content/docs/upgrading/instructions/upgrade-to-1-10-x.mdx
The website conflicts seems like it some of the changes on main didn't
make it into the 1.10.x branch, but the content is relevant for 1.10 so
I took the changes from main.
* website: migrate to new nav-data format
* website: clean up unused intro content
* website: remove deprecated sidebar_title from frontmatter
* website: add react-content to fix global style import issue