Documentation revamp

[benjaminjkraft]

I think it's probably getting time for v2 of the genqlient documentation. A few thoughts:

• It might be good to have a clearer philosophy document to explain why we don't generate a single User Go type (or why it's not great to use your gqlgen types on the client), etc.
• The FAQ is doing way more work than it should, a lot of that should become how-to guides that people might actually be able to find.
• There are probably some newer options that should make their way into said how-to guides.
• It's probably time for more examples -- or at least to make space for contributors to add them!

Specific things to fix:

• Document more clearly how to stringify large integers #128

Other ideas are welcome!