Improve formatting documentation discoverability

[iliekturtles]

Currently you need to know to dig all the way into one of the quantity type aliases (e.g. Acceleration) or view the documentation for one of the types in the uom::si::fmt module (e.g. Arguments). Add additional module level to uom::fmt and uom::si::fmt (verify this doesn't break generated code for systems besides si) as well as update uom::fmt::DisplayStyle to include an example or explicitly link to one of the above locations.

[Aehmlo]

I can take care of this.

[iliekturtles]

Make sure it is explicitly noted that std::fmt specifiers can be used.

[iliekturtles]

Per #195, in addition to making formatting documentation more visible also note the differences between formatting and get(). Also update examples to use formatting methods where appropriate.