Doc: Rework contrib appendix -- informative titles, tweaked sentences

Hi,

Attached is a patch: contrib_v1.patch

It modifies Appendix F, the contrib directory.

It adds brief text into the titles shown in the
table of contents so it's easier to tell what
each module does. It also suffixes [trusted] or [obsolete]
on the relevant titles.

I added the word "extension" into the appendix title
because I always have problems scanning through the
appendix and finding the one to do with extensions.

The sentences describing what the modules are and how
to build them have been reworked. Some split in 2,
some words removed or replaced, etc.

I introduced the word "component" because the appendix
has build instructions for command line programs as well
as extensions and libraries loaded with shared_preload_libraries().
This involved removing most occurrences of the word
"module", although it is left in the section title.

Regards,

Karl <kop(at)karlpinc(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein