Cross-compile to vendored x86_84, aarch64 (Raspberry Pi 4) (#1497)

## Issue Addressed

NA

## Proposed Changes

Adds support for using the [`cross`](https://github.com/rust-embedded/cross) project to produce cross-compiled binaries using Docker images.

Provides quite clean and simple cross-compiles cause all the complexity is hidden in Dockerfiles. It does require you to be in the `docker` group though.

## Details

- Adds shortcut commands to `Makefile`
- Ensures `reqwest` and `discv5` use vendored openssl libs (i.e., static not shared).
- Switches to a [commit](284f705964) of blst that has a renamed C function to avoid a collision with openssl (upstream issue: https://github.com/supranational/blst/issues/21).
- Updates `ring` to the latest satisfiable version, since an earlier version was causing issues with `cross`.
- Off-topic, but adds extra message about Windows support as suggested by Discord user.

## Additional Info

- ~~Blocked on #1495~~
- There are no tests in CI for this yet for a few reasons:
  - I'm hesitant to add more long-running tasks.
  - Short-term bitrot should be avoided since we'll use it each release.
  - In the long term I think it would be good to automate binary creation on a release.
- I observed the binaries increase in size from 50mb to 52mb after these changes.

book/src/SUMMARY.md

@@ -7,6 +7,7 @@
 * [Installation](./installation.md)
     * [Docker](./docker.md)
     * [Raspberry Pi 4](./pi.md)
+    * [Cross-Compiling](./cross-compiling.md)
 * [Key Management](./key-management.md)
     * [Create a wallet](./wallet-create.md)
     * [Create a validator](./validator-create.md)