Add Page: Architecture of the BDK Ecosystem

[thunderbiscuit]

This issue tracks the addition of a first page to the Architecture Section of the book.

@LLFourn had previously volunteered to write it, and I think he'd be the perfect person to do it indeed.

Here are some ideas on what I thought it could contain (just some suggestions):

1. Not about the historical "why", or how it was built in the past and why we needed to break those up. This is for another page, or maybe best for a blog post on the bitcoindevkit.org website.
2. As I see it the 9 crates break into 4 families (plus a 5th wheel):
   1. Wallets (wallet)
   2. Chain data (chain + core)
   3. Persistence (file_store + sqlite)
   4. Blockchain clients (bitcoind_rpc + electrum + esplora)
   5. Hardware signers (hwi) -> potentially being deprecated

I think it'd be great to have the page be about how (a) the 4 families come together, (b) why they are separate, and (c) why that's a good thing.

[LLFourn]

My plan would like to focus on the design of chain and reference how other things in how they connect to it.

[thunderbiscuit]

Sounds good to me!

[LLFourn]

I started working on this a bit :)