Original commit message from CVS:
Move plugin example code to new location, and put it into build system.
Add work on firstplugin chapter of pwg.
Fix typo in quotes.
Add @'s before commands in manuals.mak
Original commit message from CVS:
Added framework for writing filter writers guide.
Extracted much of the makefile from docs/manual into a shared file which
is included, so it can be re-used.