Help: Reformat cmake-generator-expressions.7 manual

Use definition lists instead of a preformatted block to enumerate
the genex documentation.

Consistently capitalize the description.

Use ``...`` to format genex parameters in the description.

Turn references to commands into links.

Add high level documentation about the motivation for generator
expressions.

Regroup expressions into subsections for different genex types. Add
a high-level description and example of the expressions in each
section.

Explode the documentation for filesystem artifacts, instead of only
referring to the variations.
Merge topic 'genex-docs'

8e729df Help: Reformat cmake-generator-expressions.7 manual