Generate the docs relative to the source protos

[ganeshs]

This PR adds support for generating the docs in the folder relative to the proto source path. If there are multiple directories containing proto files, adding the option --doc_out=markdown,index.md,source_relative will generate the file index.md under each of the proto directories with the documentation for the files in that directory

[marcoapferegrino]

I guess this commit can generate doc for proto's imports? I opened new issue because I have errors and I do not know if it is my error or is a missing feature.