Rollup merge of #40803 - lukaramu:contributing-docs, r=steveklabnik

Rollup merge of #40803 - lukaramu:contributing-docs, r=steveklabnik

Update CONTRIBUTING.md regarding documentation contributions

Changed link to documentation issues from A-docs to T-doc as this has just been migrated.
Added a link to the documentation style guidelines in RFC 1574 (I've linked to [Appendix A][1] specifically since that's the combined version).

r? @steveklabnik

[1]: https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md#appendix-a-full-conventions-text