docs(lez): Extensions section with admin-authority and extension-library guides

[mmlado]

Summary

Adds two pages under a new docs/lez/extensions/ folder, plus an "Extensions" entry in docs/lez/SUMMARY.md:

• admin-authority.md, integration guide for app developers. Covers annotation, gating, AdminCandidate (Signer / PDA), initialization window risk, transfer, renounce, IDL verification, security notes.
• build-a-spel-extension-library.md, author guide for the generic [package.metadata.spel] discovery mechanism. Covers two-crate layout, metadata fields, marker / gate macros, multi-extension stacking, common troubleshooting.

Context

These pages document the user-facing surface of RFP-001 (admin-authority library) and the framework mechanism it relies on. The framework mechanism, a generic [package.metadata.spel] extension scanner in spel-framework-core::idl_gen, is delivered as a separate PR against logos-co/spel (see companion PR). These docs are the consumer-facing complement.

Future extension libraries (e.g. RFP-002 freeze-authority) can ship without any framework change, so the "Build a SPEL extension library" page is written generically to serve as the canonical author guide for any future extension.

Why a new extensions/ folder

lez/ already groups capabilities by folder (get-started/, transfer-tokens/). Extension libraries are a new capability class. Placing both pages under lez/extensions/ mirrors the existing structure and keeps consumer and author guides discoverable side-by-side.

Companion repositories

• mmlado/spel-admin-authority: the admin-authority library implementation referenced by admin-authority.md.
• mmlado/spel:feat/admin_authority: the framework branch carrying the generic extension scanner referenced by both guides.

Open questions for maintainers

1. Folder name. Used extensions/ for brevity. Alternatives: extension-libraries/, program-extensions/, access-control/ (too narrow for the generic guide).
2. Page placement. Both under lez/. The author guide could arguably sit under build-an-app/ instead. Open to splitting if you prefer.
3. Cross-linking. The pages reference each other directly. No links from the broader lez/ or build-an-app/ READMEs yet, happy to add.

Style notes

Pages follow the existing GitBook hint-block convention ({% hint style="warning" %} for the draft-status banner and the initialization-window warning). Heading depth and table style match docs/build-an-app/tutorials/.

Relates to

• RFP-001 ([PROPOSAL] RFP-001 — Admin Authority Library rfp#46)
• Companion framework PR (when filed): mmlado/spel#feat/admin_authority