Add docs

[hecatia-elegua]

Some top-level design docs and to the Number trait, UInt.
Also, what is extract/widen useful for.

[robamu]

I think it would be a good idea to move the README.md docs to src/lib.rs and then shorten/deduplicate the README.md by adding a section like this (and adding a docs.rs badge)

# Documentation

The documentation can be found on [docs.rs](docs.rs/arbitrary-int)

what do you think about this @danlehmann ?

[robamu]

Advantages copied from PR:

This has several advantages:

1. More inline with the Rust ecosystem, where the primary ecosystem where the crate documentation is usually included in the lib.rs file and the source code
2. cargo / rustdoc can actually verify the code examples. Very useful to ensure the examples do not become stale. These tests can be run with cargo test --doc
3. Libraries which depend on / use arbitrary-int include links to the docs.rs documentation. Having the main docs on docs.rs simplifies using the library for users which have a transitive dependencies (finding the relevant docs)

Disadvantage:

One more click for README users