Add library tiers page

[thunderbiscuit]

This PR addresses issue #249.

A few discussion points:

1. We werent' sure about the core name for the tier that the bdk_wallet, bdk_chain, bdk-swift, ect. libraries are in, because the word core is so overloaded at this point (bitcoin core, also bdk_core the crate, etc.). Other ideas are: foundation, foundational, essentials, primary, tier 1.

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[reez]

should these 1-6 first letter be uppercased

[reez]

This is looking awesome and gonna be really good. just had one nit

[notmandatory]

I had a long chat with Claude 🤖 and it makes the point that the concept of the community crates is a bit orthogonal in that these could be stable or experimental but are maintained by the community and not the foundation. Suggested text:

BDK Library Classification

All BDK libraries are classified along two dimensions:

Maturity Level

• Stable: Production-ready with comprehensive testing, documentation, and backward compatibility guarantees
• Experimental: Early-stage, APIs may change, minimal guarantees

Support Model

• Foundation: Maintained by volunteers and grant-funded developers from the BDK Foundation
• Community: Maintained only by volunteer contributors

[Then you'd have four sections explaining what to expect from each combination]

[notmandatory]

I also think this page belongs as a header and footer level link instead of going into the foundation page. It could still have a link to the foundation where appropriate.