configure.ac: use 5-argument AC_INIT() and also define PACKAGE_DESCRIPTION
doxygen/Doxyfile.in: new

this is the verbatim output of "doxygen -g".
doxygen/DoxygenLayout.xml: new

this is the verbatim output from "doxygen -l".
generate doxygen/Doxyfile and export a DOXYGEN_INPUT_FILES variable
doxygen/Doxyfile.in: add our changes

note that the initial commit for this file contains the verbatim
"doxygen -g" output from doxygen 1.8.6, so by implementing our changes
in a subsequent commit we get a precise record of what we changed from
the default template doxygen config file.

also note that any project-specific settings get substituted in via
autoconf (doxygen/Doxyfile is listed in AC_CONFIG_FILES), so this file
can be reused verbatim in other projects.
doxygen/DoxygenLayout.xml: move the detailed description on group pages up
add 'Introduction' and 'Library overview' documentation
doxygen/Doxyfile.in: enable BRIEF_MEMBER_DESC
doxygen/Doxyfile.in: set QUIET = YES

doxygen's output is very verbose. turn off most of the output.
add API documentation
fs_io_options_validate(): fix error message for iovec_size validation: 'multiple' -> 'power'
move doxygen/* to the root of the repository
add doxygen html output to the build system
.commit_docs.sh: new