Write `piladb` complete documentation

[fern4lvarez]

piladb needs proper documentation.

The chosen approach is to use GitBook, as it provides Git/Github compatibility, Markdown, well structured sections, nice user interface and possibility to export to PDF, Mobi or ePub.

The project will be called docs.piladb.org.

GitBook site: https://www.gitbook.com/book/fern4lvarez/docs.piladb.org Github repository: https://github.com/oscillatingworks/docs.piladb.org