docs: provide links to what's supported on gitbase

[erizocosmico]

It could be nice for the users to link directly from engine docs to the following things:

• schema (https://docs.sourced.tech/gitbase/using-gitbase/schema)
• functions (https://docs.sourced.tech/gitbase/using-gitbase/functions + https://github.com/src-d/go-mysql-server/tree/3dd441325d1731821eff0495fbf63747c258b8ff#custom-functions)
• supported SQL syntax (https://docs.sourced.tech/gitbase/using-gitbase/supported-syntax)

[carlosms]

Related to #153.
At some point it was said that the docs in Engine should be self-contained without links to the other projects.
I personally prefer to have a few examples here and link to the gitbase docs that @erizocosmico posted.

@marnovo can you comment on this?