This includes the target property, its initializing variable, its
initializing environment variable, and updating related docs to mention
the new bits.
Define the entry point to each mode as an option for the `cmake`
program, but reference the options for that mode as part of stand-in
`cmake-<mode>` programs.
Add some prose to the documentation index page to guide readers to the
major manuals and guides.
Add a new "User Interaction Guide" to help the class of new user
who wishes to build a project with CMake for the first time, such as
after cloning a repo from a git repository.
Add a new "Using Dependencies Guide" to help the class of new
user who wishes to consume a SDK provided by a third party and needs a
starting point. This is a different type of user to the user who wishes
to create their own project from scratch (addressed by the
`cmake-buildsystem(7)` manual) as each will encounter needs for
information discovery in a different order.
