Help: Move reference content to its own section at end of genex manual

This involves moving the Debugging section up to just after the
introduction. The content of that section is unchanged.
The reference section then follows, and since it is a dedicated
section, all existing sections thereunder were demoted one level.

Move the note about deviating from the usual CMake docs convention
of using angle brackets around placeholders out of the Introduction
and to the start of the reference section. No placeholders are used
before that point, so that content no longer belongs in the introduction.
Apply some minor wording cleanup to the content in the Introduction
that remained.

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