Signed-off-by: David Goulet <dgoulet@torproject.org>

This doesn't actually do the right thing: it will make somebody get
ignored completely for a release if they _only_ use their
alternative name.

Disclaimer: I have no idea what I'm doing when it comes to shell
scripts.

maint/thanks.sh autogenerates a list of contributors since a revision
(that you pass it on the command line), intended to make the work of
going through the shortlog a bit less manual. It can exclude
contributors' names given in maint/exclude_contributors.txt, in order to
filter out people who work for Tor, or who commit under more than one
name.

Most of the structs in `arti-client` have example code now, to give a
clearer idea of how they're used.

Annoyingly, a lot of the types exposed in `arti-client` are actually
re-exports, which makes documentation a bit harder: example code that
references other parts of `arti-client` can't actually be run as a
doctest, since the crate it's in is a dependency of `arti-client`.

We might be able to fix this in future by doing the documentation in
`arti-client` itself, but rustdoc seems to have some weird behaviours
there that need to be investigated first (for example, it seems to merge
the re-export and original documentation, and also put the re-export
documentation on the `impl` block for some reason).

For now, though, this commit just writes the docs from the point of view
of an `arti-client` consumer, removing notes specific to the crate in
which they're defined. It's not ideal, but at least the end user
experience is decent.

Instead of putting a fully qualified name in the text, in most cases
we should just use the short name of the type or function we're
referring to.

In other words, instead of saying [`crate::module::Foo`], we should
typically say [`Foo`](crate::module::Foo).

I'm not 100% sure this is better, but it might help the user
understand how Arti works a bit better.

This overhauls the top-level `arti-client` documentation significantly:

- the "Using arti-client" section walks the user through all of the
  necessary steps to initiate a Torified TCP connection, and then
  provides a code example
  - this example is also available as `examples/readme.rs`; it's not run
    as a doctest, since it involves connecting to Tor
  - a "More advanced usage" subheading provides information about stream
    isolation (and can potentially be used for other interesting
    features once we get them).
- a new "Multiple runtime support" section was added to explain the
  purpose and usage of the `tor-rtcompat` crate
- the section on design and privacy considerations was removed; this is
  probably okay to keep in a README, but users of the crate aren't going
  to be interested in this (at least I don't think)

(also, the doc comment for `arti_client::Error` was fixed to make actual
sense)