doc: Add some high level notes about rust-miniscript design

[tcharding]

Add a file to doc/ giving some high level notes about the design of the repo. This is Sanket's work picked up from #603.

[tcharding]

The only changes to the original PR are the review suggestions I made back then. cc @sanket1729 just FYI.

[trevarj]

Nice. Would like to see one bullet describing the Miniscript type and another bullet describing the Descriptor type. Maybe another bullet on how they relate?

[tcharding]

Fair. Will require some thought if I'm to write those.

[apoelstra]

I don't think we want to foreground the two types of policies. We should definitely cover Descriptor and Miniscript and their relationship. We should also talk about Taproot and maybe the other kinds of descriptors (and why e.g. Descriptor::address returns a Result).