skip "module" not found as plugin, id est directory

If PAGER is set, or the executable less is present, ansible-doc will use
it to pipe information into so that it can be scrolled through.
If the environment variable LESS is not set, this will set it to FRSX.

And also be liberal in what ansible-doc accepts as an empty
requirements specification.

While here fix a typo in template module documentation
wrt jinja2 overrides example.

tweak position & look of Requirements in rst
tweak APT's notes as per anhj's wish

return DOC and EXAMPLES as a list
add moduledev explanation
more

check path is directory
added manpage & setup
small cleanup
shut up module_formatter in utils to avoid trace print on crud files in library