[Checkers] "How to create a new checker plugin" section in Checker Framework manual

[Michael Ernst]

I've edited the "How to create a new checker plugin" section in the Checker
Framework manual.  I wanted to bring up two themes that I noted multiple times.

1. I eliminated redundancy.  In many places, the same information was being
given in a single section.  Sometimes this became most apparent after
fixing the section's structure to put all the information in a consistent
and logical order.

2. I organized the manual by tasks rather than by mechanism, but made the
mechanisms clear.

The most important thing was to take the single section on
meta-annotations, and move the description of each meta-annotation into the
section that discussed a particular task.  (Recall that I made the same
change to the Checker Framework paper; I was surprised to see this
organization re-appearing in the manual.)  That is a more logical place to
put them, as a user has a particular task in mind rather than wanting a
full list of all the meta-qualifiers.

I also split the sections that discussed a particular task (declaring the
type hierarchy, declaring the implicit annotations) into subsections that
discussed how to perform the task declaratively and how to perform it
procedurally.  This makes it clearer how to do the task and what the two
approaches are.

                    -Mike