Merge bitcoindevkit/bdk#1592: Architectural Decision Records

3b10abbef69a2245714a762a555ff58a67217eec docs: add ADR 0002_persisted.md (valued mammal)
1c4b24469a971fdb2490b7358b10f9c480a7c16a docs: add ADR 0001_persist.md (valued mammal)
e60c65b484fb8a3754b206b97cc3ef4a4d2f6334 docs: add architectural decision record (ADR) template (valued mammal)

Pull request description:

  Opening this up as a record of past decisions. The template is inspired by the one used by uniffi. Whether or not we decide to add these to the repo, it's an opportunity to learn, discuss, and collab when it comes to design and architecture.

  The first 2 ADRs pertain to bdk's approach to data persistence. Another recent suggestion was to document how we're thinking about single vs multiple descriptor wallets. Feedback welcome.

  fixes #1309

ACKs for top commit:
  thunderbiscuit:
    ACK 3b10abbef69a2245714a762a555ff58a67217eec.

Tree-SHA512: 9eeed764a21805b00189320efa6266fd77c333e530295df8b3bab7d97400a8658db712a0c4b17ea056355f5ff2c110f74e0e384b85a71489bbba1fce1a19b7d4