GitLab

Useful for debugging, but should not be used in production.

Fixes #4.

Add a link to our documentation on `docs.rs` using the semver parsing
feature documented at <https://docs.rs/about>.

Remove the `.git` extension from the GitLab repository link.  Git can
still use the URL, but now browsers will be shown the human-readable
page when a human follows the link from `crates.io`.  Drop the homepage
link because that is now the same as the repository link.

0.2.24 uses serde 0.8 which I haven't updated to yet.

Got bored typing up docs part of the way through. Some of the more
obscure entities are left untouched.

Objects meant to be passed to POST or PUT endpoints are not represented
here (though they should be added).

Fixes #1.