Relay docs missing from guides TOC

[jpaas]

The links to the relay docs in the Guides TOC https://graphql-ruby.org/guides#relay-guides seem to be missing

[rmosolgo]

Thanks for pointing this out. I removed most of them (Connections, Mutations) because the Relay:: constants are deprecated, but I forgot to remove that section from the index page.

In fact, I should really leave it there, and add a RangeAdd doc. I'll keep this open til I get a chance to add that doc.