Maintain a list of the supported BIPs in the repository

**Describe the enhancement**

Inside a doc directory at the root of the repository, or maybe in the header section of the project, we could list the BIPs the library is compatible with, or those that are partially supported, and detail the missing components.

**Use case**

Potential users of the library may want to check before starting building on top of BDK which BIPs are supported by the project.

**Additional context**

I was reading proposed [BIP 3](https://github.com/bitcoin/bips/blob/master/bip-0003.md) updated BIP process, and the [Adoption Of Proposals](https://github.com/bitcoin/bips/blob/master/bip-0003.md#adoption-of-proposals) section encourages Bitcoin related projects to publish a list of the BIPs they implement. As a reference, bitcoin core has one: [doc/bips.md](https://github.com/bitcoin/bitcoin/blob/master/doc/bips.md). Although the state of BIP 3 is not yet "deployed", I think it is in general a good idea.

Maybe this doesn't make sense here but on upstream libraries, or maybe on both. Here I'm just gathering feedback.

**Impact**
- [ ] Blocking production usage
- [x] Nice-to-have / UX improvement
- [x] Developer experience / maintainability

**Are you using BDK in a production project?**
- [ ] Yes
- [x] No
- [ ] Not yet, but planning to

**Which backend(s) are relevant (if any)?**
- [ ] Electrum
- [ ] Esplora
- [ ] Bitcoin Core RPC
- [x] None / not backend-related (e.g. `bdk_chain`, `bdk_core`)
- [ ] Other (please specify): `____`

**Project or organization (optional)**