Revamp the README and the generated docs

[gdubicki]

I have updated this projects docs in a similar style to voxpupuli/puppetboard#601 . The main changes are:

• Moved the user guide out of the README and into the /docs so it will be served from readthedocs.io,

• Reorganized the docs into 3 main sections: user guide, connecting from other hosts than the puppetdb's, developer guide,

  • Unfortunately I don't have the knowledge to update the developer guide one, so I added a warning that it may be out of date,

• Changed the docs theme to a more modern one - the one used by https://toolbelt.readthedocs.io , for example,

• Cleaned up and updated the README,

  • Removed the info about the native packages from the README as they either don't exist or are outdated + removed the outdated .spec files,

• Changed the format of the README and changelog from RST to Markdown, to make them for friendly to contributors,

• Moved the contributing guide into a separate file and updated it,

[gdubicki]

This is how the docs will look: