Set up the Sphinx source directory

After several months postponing this task, the time has finally come
to document LEMON — we are slowly getting dangerously close to the
commissioning of PANIC and a proper user guide is a must. We will use
Sphinx, of course, an elegant weapon venerated by Python developers.

This commit is just the result of running sphinx-quickstart to create
the documentation source directory, Doc, with the configuration file
conf.py and a master document, index.html, which serves as a welcome
page and to contain the root of the Table of Contents tree.
Add the first section of the documentation of the 'import' command

I have started with the "A historical note" section, which describes
the origins of this LEMON command and explains why we needed it when
reducing images taken at the 1.23m CAHA. Although the first written,
this will probably be one of the last sections, as most users will not
even be interested in reading it — but this is something that I have
wanted to write down for a long time.
Add "Usage" section to the documentation of 'import'
Add "Overview" section to the documentation of 'import'
Add "Options" section to the documentation of 'import'
Add "FITS keywords" section to the documentation of 'import'
Add "Examples" section to the documentation of 'import'
Use the 'option' role to cross-reference command-line options

Note that we are removing the 'program' directive to avoid having to
qualify the references in the option roles by the program name. All
the commands documented here refer to the 'import' command and thus
there is no need to differentiate between programs.
End directory names with a trailing slash
Add Javier Blasco