Document a quickstart for fpm and GFortran using Conda on all platforms

[milancurcic]

Originally from proposed by @certik here.

Document on fortran-lang.org how to get started on each platform using Conda in 5 minutes. Conda is nice because it does not require root access and installs locally, in a non-invasive manner. And the usage from the user perspective is almost identical on all platforms.

[LKedward]

And the usage from the user perspective is almost identical on all platforms.

Note that fpm isn't yet available on Windows via Conda

[awvwgk]

I have a quite complete setup for Ubuntu, MacOS and Windows here. It's not a complete conda setup because I prefer to use the system compilers, but it's close.

For (native) Windows I made good experience with MSYS2, a few students chose this over the WSL approach and were quite successful.