doc: add a CSS file
doc,build: add a 'doctest' target

This `make` target build a small subset of the documentation (for
testing purposes).
doc: add an ocamldoc plugin (with {figure _})
doc: improve the CSS
doc.biohtml: add links to figures
doc.biohtml: implement “foldable included modules”
doc.biohtml: overload html_of_module_type_kind
doc.biohtml: display modules inline
doc.biohtml: generate less files
doc.biohtml: put class definitions also inline
doc.biohtml: do some code/build clean-up
doc: add figures for the Transform API
doc: improve documentation of the Bam module
doc: change headers levels in intro.txt
doc: improve CSS
doc: improve documentation of Bed's API
Fasta: improve and document API

The improvements are just making sure that error types in Result.t are
“open”.
doc.{biohtml,CSS}: add highlighting for elements
Fastq: fix API documentation
Gff: improve documentation