docs: add explanation of --disable-git command (#80882)
The `--disable-git` command was implemented in #68821 (released in
`v15.0.0-canary.119`).
It was documented in create-next-app/README.md, but not in the website.
I've seen several discussions online about the feature (usually from
people who remember an old `--no-git` flag) so this will be nice to
publish.
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
