Simplify and correct the Airnode book

[saviro-orin]

Summary

• reorganize the sidebar around reader tasks: run, use, understand, optional features, reference, and security
• rewrite the introduction, endpoint ID, and trust model chapters in plain language
• remove claims that signatures or TLS proofs establish more provenance than they actually do
• correct endpoint ID secret handling, key rotation, startup order, replay scope, and timestamp descriptions
• replace the unsafe minimal callback example with checks for verifier, signer, endpoint, and freshness
• fix broken repository links and smaller command/reference inconsistencies

Overall, this removes 135 net lines while preserving the useful reference material.

Verification

• bun run fmt
• bun run lint:prettier
• bun run lint:eslint
• bun run book:build

The Docusaurus build succeeds. It retains the existing vscode-languageserver-types dynamic-require warning.