docs mdbook

[kagarmoe]

Resolved issues:

First commit from docs to the s2n mdbook, mostly of the "getting to know mdbook" variety.

Description of changes:

1. Expanded entries on the summary
2. Added search (A nicely implemented search, btw!)
3. Set mdbook to use 2021 rust, which will be relevant for playground code
4. Added an example directory with a few examples, mostly to see how it works

Call-outs:

There's some shortcomings in how the includes work. It doesn't look like I can hide a range of lines from the middle of a code example. For example, hide lines 5-10 of a 25 line file.

Testing:

Standard testing.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[WesleyRosenblum]

We follow the Conventional Commits guidance for commit messages. In this case, the title should be something like docs: initial mdbook edits or something similar

[kagarmoe]

Good to know & completely undocumented.