mbed new --scm=none

This is merely a matter of documentation:
whereas in docs of new command, we read, that --scm may apply values
- git (default),
- hg, or
- bld (never heared of that),
code shows that also value none is allowed.

Value none is important if mbed new shall create a program which is within a more comprehensive git project. The value none really occurs in the code and so it shall be documented in the help.
Also i feel that in the internet site
https://os.mbed.com/docs/mbed-os/v5.15/tools/working-with-mbed-cli.html
this value shall be mentioned,
as the application case is important.

Maybe also a good idea to document --scm more generally:
what blk is and that hg and git can be used transparently.

Another aspect: it is not ```mbed new --scm none .``` but it is ```mbed new --scm=none .```
which should also be reflected in the documentation.