Add Documentation Quadrant (Diátaxis framework) to mkdocs

[Copilot]

The docs were ported to mkdocs but lacked real content across all four documentation types. This PR flushes out the Diátaxis quadrant — Tutorials, How-to Guides, Reference, and Explanation — with substantive new pages.

Tutorials (docs/tutorials/)

• getting-started.md — Install → create a key → derive an address → BIP32 → ku CLI
• first-transaction.md — Coinbase tx → spendables → unsigned tx → sign → serialize

How-to Guides (docs/how-to/)

• bip32-keys.md — Seed → xprv/xpub export, child derivation, public-only (watch-only) wallets, BIP44/49/84 paths
• sign-a-transaction.md — Single WIF, multi-WIF, BIP32 node, Keychain, create_signed_tx, partial multisig, GPG-encrypted key files via tx CLI
• multisig.md — Full 2-of-3 walkthrough: keys → redeem script → P2SH address → fund → incremental signing → who_signed

Reference

• cmdtools.md — Replaced one-line stub with complete ku + tx reference: all options, env vars, and annotated examples (inlined from COMMAND-LINE-TOOLS.md)

Explanation (docs/explanation/)

• design-philosophy.md — Network-centric API model, pycoin vs Satoshi naming conventions (puzzle_script/solution_script vs scriptPubKey/scriptSig), key class hierarchy, multi-network support, ECDSA acceleration backends

Navigation

• mkdocs.yml nav restructured into four labelled sections matching the quadrants
• docs/index.md replaced with a quadrant overview table and quick-start snippet

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