[ty] Add warning that docs are autogenerated (#18270)
## Summary
This is a practice I followed on previous projects. Should hopefully
further help developers who want to update the documentation.
The big downside is that it's annoying to see this *as a user of the
documentation* if you don't open the Markdown file in the browser. But
I'd argue that those files don't really follow the original Markdown
spirit anyway with all the inline HTML.
