Revisit build commands

[chalin]

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.)