Add 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:

• Setup the build environment for the documentation,
• Write an introduction including an explanation about the project and install instructions,
• Document the command line interface, e.g. man pages for the photo-idx script and its subcommands,
• Document the GUI,
• Reference documentation for the Python API,
• Setup a RTD site,
• Update README and setup script, including pointers to the new RTD site.

Close #2.