Document no std support

[notmandatory]

Describe the enhancement

The bdk and other workspace packages default to std support but should be able to be built for no std targets. Rather than complicating the cargo features setup with a no-std feature we should create separate documentation in the repo to demonstrate how to use bdk in a no std target project.

Use case

This is useful for anyone building bdk for a no std target system such as a micro controller or even WASM which has incomplete std support.

Additional context

See discussion in #894