Add build environment for the documentation

[RKrahl]

Add documentation.

Note: this is not yet done, hence this is a draft PR. I open it now in order to structure the work and to formulate a task list:

• [x] Setup the build environment for the documentation,
• [x] Write an introduction including an explanation about the project and install instructions,
• [ ] Document the command line interface, e.g. man pages for the archive-tool and backup-tool scripts and their subcommands,
• [ ] Document the configuration file for the backup-tool.
• [ ] Reference documentation for the Python API,
• [x] Setup a RTD site,
• [ ] Update README and setup script, including pointers to the new RTD site.

Close #7.