Restructure documentation and add a guide on writing reporters

[arkgil]

This changes a few things in the documentation:

• documentation in the Telemetry.Metrics module has been shortened to only provide a quick information to get started + documentation of all the functions and possible options
• three documentation pages have been added
  • overview, which provides a longer, high-level description of the library and its "ecosystem"
  • rationale
  • a guide for writing reporters

I hope this will make it clearer for newcomers what all of this is actually supposed to do 😄

/cc @bryannaegele

[arkgil]

When reviewing, I recommend running mix docs locally to get a feel of new structure 🙂

[josevalim]

Sounds good :) --

José Valimwww.plataformatec.com.br http://www.plataformatec.com.br/Founder and Director of R&D