doc: make TOC scroll properly
doc: use better fonts, minor layout changes

halmd.css_t: add missing style "p.rubric"
doc: minor changes
doc/readme: mention Nicolas Höft explicitly
doc: include develop.rst in TOC tree

Let the developer guide start with this overview.

Rename directory developer/ to develop/
doc: revise section on H5MD output files
doc: update section "Getting started"