Improve the wasmtime crate's README (#4174)

* Improve the `wasmtime` crate's README

This commit is me finally getting back to #2688 and improving the README
of the `wasmtime` crate. Currently we have a [pretty drab README][drab]
that doesn't really convey what we want about Wasmtime.

While I was doing this I opted to update the feature list of Wasmtime as
well in the main README (which is mirrored into the crate readme),
namely adding a bullet point for "secure" which I felt was missing
relative to how we think about Wasmtime.

Naturally there's a lot of ways to paint this shed, so feedback is of
course welcome on this! (I'm not the best writer myself)

[drab]: https://crates.io/crates/wasmtime/0.37.0

* Expand the "Fast" bullet a bit more

* Reference the book from the wasmtime crate

* Update more security docs

Also merge the sandboxing security page with the main security page to
avoid the empty security page.

docs/SUMMARY.md

@@ -48,7 +48,6 @@
   - [Wasm Proposals Support](./stability-wasm-proposals-support.md)
 - [Security](security.md)
   - [Disclosure Policy](./security-disclosure.md)
-  - [Sandboxing](./security-sandboxing.md)
 - [Contributing](contributing.md)
   - [Architecture](./contributing-architecture.md)
   - [Building](./contributing-building.md)