docs: move support policy (#10356)
### Description
The Support Policy page was nested into "Getting Started", which felt
like an odd place for that bit of information. This PR moves it out to
the top level of the information hierarchy.
### Testing Instructions
-
[turbo-site-git-shew-035b4.vercel.sh/docs/support-policy](https://turbo-site-git-shew-035b4.vercel.sh/docs/support-policy)
should work.
-
[turbo-site-git-shew-035b4.vercel.sh/docs/getting-started/support-policy](https://turbo-site-git-shew-035b4.vercel.sh/docs/getting-started/support-policy)
redirects to the new path.
- The link to it in the page is now in the top level of the sidebar.
