Add docs for the global config file, registry, manifest changes and publish

[minhqdao]

Add docs for https://github.com/fortran-lang/fpm/pull/817 and https://github.com/fortran-lang/fpm/pull/876:

• [x] How to use a global configuration file.
• [x] How to setup a local registry.
• [x] How to use a remote registry.
• [x] Additional manifest options (namespace and v).
• [x] How to upload a package to the official registry using fpm publish.

[minhqdao]

I would still considering adding a note warning regarding this first alpha version of registry. Overall the PR looks good to me.

Good point, it's been added.

[minhqdao]

I'd appreciate if at least one native English speaker could review this. @everythingfunctional?

[minhqdao]

Thank you for your reviews @everythingfunctional @arteevraina @henilp105