More tutorials for using fpm

[awvwgk]

We need more tutorials to cover different aspects of fpm. Here are some ideas:

• [x] adding dependencies: https://github.com/fortran-lang/fpm-docs/pull/18
  • build a new project that depends on stdlib
  • add development dependencies and build unit tests with a testing framework with test-drive/vegetables
  • build a command line application with M_CLI2
• [x] using fpm plugins: https://github.com/fortran-lang/fpm-docs/pull/36
  • installing fpm-search with fpm
  • querying the registry
  • adding a dependency from the registry

[band-a-prend]

There isn't tutorials about passing compiler command name, environment variables/compiler flags that only are discribed here. At least I could find it under "Tutorial" or "Reference" sections.