Revisit build commands

Context: https://github.com/grpc/grpc.io/pull/561#discussion_r544440860

> We don't want to fetch the latest `docsy` repo version (which is what the `git submodule update ...` command does AFAIK), we want to stick to the submodule version that is associated with this repo via `.gitmodules`. Otherwise a build on Netlify might (unknown to us) be different from local builds.
>
> AFAIK, it shouldn't be necessary to add this extra `git` command (unless the `docsy` repo itself has submodules).

This needs to be further investigated. Currently, the modified build commands are part of `netlify.toml`. Some refactoring my be in order, including a shift of commands towards the `Makefile`. (I'd actually vote for dropping make in favor of simple npm scripts (if those are powerful enough), or possibly gulp.)