[Documentation] Modify repo structure to render it using GitHub pages

[alpgarcia]

As a first step towards having collections of panels, @jsmanrique proposed to document repositories using the following structure:

.
├── docs
│   ├── assets
│   │   └── images
│   ├── panels
│   ├── _config.yml
│   ├── index.md
│   └── panel.md
├── panels
├── CONTRIBUTING.md
├── LICENSE.md
└── README.md

Beyond the obvious .md files:

• panels has the set of .json files for all the panels.
• docs folder has an structure to be rendered as static web with Jeckyll (GitHub does it directly):
  • _config.yml is the config file for the Jekyll page, and it must indicate that there is a collection of items in panels. See collections docs for Jekyll
  • index.md is the default website the user will see in bitergia.github.io/{panels-collection-name} (as we have https://chaoss.github.io/grimoirelab, that is rendering the content in https://github.com/chaoss/grimoirelab/docs)
  • panel.md is the default template for a panel documentation.
  • assets/imageshas panels screenshots.
  • panelshas one .md file for each panel that document the metrics and charts in that panel.

This is related to #296

[alpgarcia]

Closed by #303