dist: no need to specify anything from doxygen-doc/ under doc/Makefile.am

doxygen-doc/ is manually copied by config/distscript.csh

Therefore, let config/distscript.csh handle failure to create the doc
when doxygen is missing.

By the way, update obsolete comments about adding entire directories
to EXTRA_DIST.
doc/Makefile.am: factorize AM if/endif and remove obsolete stuff
doc: further simplify doc/Makefile.am

When doxygen and readme are buildable, we always build them.
When not, we just do nothing and let config/distscript.csh fail later.
doc: build the top-level README as a single rule

2 rules with intermediate file being removed means
that those rules always rerun.
doc: only try to copy doxygen.css when the doc was rebuilt