Help: Document the CMAKE_TOOLCHAIN_FILE.

Link to the new docs from existing references to the variable.
Help: Document the CMAKE_FIND_ROOT_PATH* variables.

Add a replacement template for the variables, and link to them from
the documentation for the find_* commands.
CTest: filter /showIncludes output from ninja compile launcher

Teach "ctest --launch" a new "--filter-prefix" option.  Set it using
the CMAKE_CL_SHOWINCLUDES_PREFIX value with the Ninja generator.
Help: Add cmake-toolchains.7 manual

Create a manual for toolchain related docs.
Merge topic 'cmake-toolchains-manual'

c45b5ce Help: Add cmake-toolchains.7 manual
3e3f8b4 Help: Document the CMAKE_FIND_ROOT_PATH* variables.
531c108 Help: Document the CMAKE_TOOLCHAIN_FILE.
Merge topic 'filter-showincludes-in-launcher'

e5e3f3d CTest: filter /showIncludes output from ninja compile launcher