[docs] Moving "Codegen for gRPC" to configuration category

Summary:
Moved "Codegen for gRPC" to configuration because it contains 'brew' and other commands associated with config.

This helps the Nix homie not have to cross-reference everything.

Test Plan: N/A

Reviewers: atul, palys-swm, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, benschac, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2357