Help: Add cmake-developer.7 manual

Add the manual with just an introduction section.  Leave section headers
for Help and Modules to be filled in later.
Help: Move Modules/readme.txt content into cmake-developer.7 manual

Move all content from Modules/readme.txt except for the link to the
module maintainers wiki page into "Help/manual/cmake-developer.7.rst".
This produces some invalid reStructuredText markup to be fixed in a
future commit.
Help: Fix module documentation markup in cmake-developer.7 manual

Convert the content moved from Modules/readme.txt to valid
reStructuredText markup.  Mainly, convert the lists of variables to
definition lists, wrap long lines in paragraph text, and add literal
block markup and indentation.
Help: Organize cmake-developer.7 manual Modules section

Add "Module Documentation" and "Find Modules" subsections.  Add to
Modules/readme.txt a textual reference to the cmake-developer.7 manual
and, while at it, fix the wiki URL domain.
Help: Add documentation format to cmake-developer.7 manual

Document how CMake uses reStructuredText to provide the help manuals.
Cover supported inline markup and directives, the CMake Domain in
Sphinx, and cross-reference syntax.
cmRST: Cross-reference cmake-developer.7 manual and cmRST Doxygen

The cmRST implementation and the list of capabilities documented in the
cmake-developer.7 manual must be kept in sync.  Add a note to each file
to reference the other.
Merge topic 'doc-cmake-developer' into next

e0ce6f0 cmRST: Cross-reference cmake-developer.7 manual and cmRST Doxygen
c1c239b Help: Add documentation format to cmake-developer.7 manual
a06a90f Help: Organize cmake-developer.7 manual Modules section
28f08ba Help: Fix module documentation markup in cmake-developer.7 manual
baaab06 Help: Move Modules/readme.txt content into cmake-developer.7 manual
80a311e Help: Add cmake-developer.7 manual