Help: Mention INTERFACE_SOURCES as settable for INTERFACE libs

The add_library(INTERFACE) and Interface Libraries documentation
list all the INTERFACE_* properties and target_* commands that
can be used to define the interface.  Add INTERFACE_SOURCES
and target_sources() to these lists for completeness.
Help: Organize Binary Targets section of cmake-buildsystem.7

Add a subsection for Binary Executables just before Binary Library
Types.  Divide the library section into Normal Libraries and Object
Libraries.
Help: Update style guide to use section headers for command signatures

In order to be able to link to specific command signatures we need to
use a section header instead of a horizontal separator.
Help: Organize add_library command documentation

Add a section for each library type signature.  Add a table of contents
at the top for the html builder.
Help: Link to Object Library docs from add_library
Merge topic 'buildsystem-doc-updates' into next

20f54602 Help: Link to Object Library docs from add_library
a8153181 Help: Organize add_library command documentation
d8319f0f Help: Update style guide to use section headers for command signatures
50dca471 Help: Organize Binary Targets section of cmake-buildsystem.7
4054534c Help: Mention INTERFACE_SOURCES as settable for INTERFACE libs