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.
Many environment variables were documented late and got
assigned wrong versions by the script.
(The whole Help/envvar section was only added in 3.10).
Issue: #19715
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.