In each environment variable doc page, insert an opening
paragraph (ENV_VAR.txt) to say that this is an environment
variable, and provide a link to the cmake-language section
on environment variables.
Add documentation for both the CMake variable and environment variable
of this name pattern. Update references to these names to link to their
documents. Clarify the pattern used to construct their names.
While we already support `cmake --build . -- -j`, the options after `--`
are specific to the native build tool. Add new options `--parallel
[<N>]` and `-j [<N>]` to abstract this and map to the proper option
for the native build tool.
