Help: Use "^^^^" for subsubsection headers

This is the convention suggested in the Sphinx documentation and is
already used in several other places in the CMake documentation.
Update a few places where we were using other characters.
Help: Add documentation style section headers to cmake-developer.7

Give the style guides titles instead of numbers so we can link to them.
Help: Organize documentation style sections in cmake-developer.7

Now that the style guidelines have section titles instead of numbers,
organize them into more well-defined sections.
Help: Document section header underline hierarchy in cmake-developer.7

Explicitly specify the sequence of underline characters we use in the
CMake documentation.  It is the same sequence as that suggested in the
Sphinx documentation, but we have our own descriptions.
Merge topic 'doc-section-header-convention'

793b64e4 Help: Document section header underline hierarchy in cmake-developer.7
05bd31ab Help: Organize documentation style sections in cmake-developer.7
eaafe756 Help: Add documentation style section headers to cmake-developer.7
4207b3a3 Help: Use "^^^^" for subsubsection headers