Port Graal's documentation to chaoss/grimoirelab-graal

The main aim of this issue is to port graal's documentation from the [tutorial](https://github.com/chaoss/grimoirelab-tutorial/tree/master) to [chaoss/grimoirelab-graal](https://github.com/chaoss/grimoirelab-graal).
https://chaoss.github.io/grimoirelab-tutorial/graal/intro/
https://github.com/chaoss/grimoirelab-tutorial/tree/master/graal

This should be done similarly how perceval documentation is ported to [chaoss/grimoirelab-perceval docs](https://github.com/chaoss/grimoirelab-perceval/tree/master/docs/perceval).
1. Open a PR in chaoss/grimoirelab-graal adding the content, get it merged
2. Open a PR to remove the files from master branch, adding the link of the docs in the commit message