[Rigidity]

The Sage pages were written around v0.10/0.11, and some of what they said no longer matched the code. This brings them in line with v0.13.1, the release src/openapi.json is synced from.

• Config: wallets.toml no longer has the change and derivation modes (defaults.change, defaults.derivation or the per-wallet overrides), so those sections are gone. Added the per-wallet emoji and change_address fields, with a warning that Sage doesn't check that a change address belongs to the wallet. Fixed the peer_introducers example, which used the dns_introducers key.
• Files: the log directory is log, not logs. Added apps, sage-apps.sqlite3 and themes, and described the wallet database layout (wallets/<fingerprint>/<network>.sqlite).
• RPC setup: the cargo install command now uses v0.13.1 instead of v0.11.1.
• Getting Started: links to the Sage Apps guides on fancybudgie.com.

Features that are on Sage's main but not in a release yet (watch wallets, XCHandles config, SAGE_ROOT, the sage wallet CLI commands and others) are left for a follow-up once they ship.

pnpm build passes.