Add .gitignore's.
Add multind.sty.

This style is missing even from Debian's texlive-latex-extra.
Unbreak HTML ocamldoc.

This commit properly runs local ocamldoc via an explicit ocamlrun
invocation, as already done in manual/library/Makefile.
Add a manual section on compiler-libs.
Update the description of -ppx in ocamlc and ocamlopt chapters.