docs: better documentation for silent make rules

docs: better documentation for silent make rules

* doc/automake.texi (Options): Detailed description of the
automake option `silent-rules' moved from here ...
(Silent Make): ... into this new chapter, expanded, improved,
and subdivided into ...
(Make verbosity, Tricks For Silencing Make,
Automake silent-rules Option): ... these new sections.
(@menu, @detailmenu): Update.
* tests/silent-configsite.test: New test, checking that the
user can control default mode of silent-rules from config.site,
as is documented in the manual.
* tests/Makefile.am (TESTS): Updated.