Fix documentation and add documentation for elixir

[D4no0]

I would like to make a MR, where I will fix the documentation build, add possibility for documentation to be built for HexDocs just like telemetry, and finally try to complete all the missing documentation.

What I need to know is how _bhvr files were generated and why they contain the invalid @doc tags?

[tsloughter]

Great!

The files are generated by the plugin based on templates in its repo https://github.com/tsloughter/grpcbox_plugin/tree/master/priv

[D4no0]

I've create a MR to fix documentation build: #82 . After this is merged I can add the scaffolding for hexdocs html documentation generation.

[D4no0]

I added scaffolding and configuration for generating documentation on HexDocs : #83

[D4no0]

At this point, after merging these 2 PR's, you should be able to build/publish the documentation of the project to HexDocs. The next step would be to make examples, document functionality for both elixir and erlang.