Add pages site with Gitaly protobuf docs

The old gitaly-proto published a GitLab Pages site with documentation for the Gitaly protobufs. This is useful for users outside of the Gitaly team to better understand RPCs without digging into the repo.

Migrating this functionality over to the Gitaly repo by adding a target to the Makefile to build HTML documentation for our protos and a pages job to publish them.