docs: add install step, migration note, and typed try example

[jog1t]

• Quickstart: add an ## Install section (npm install @rivet-dev/workflows) and note that the package re-exports RivetKit, so setup, actor and queue share the import while clients keep rivetkit/client.
• Introduction: name workflow({...}) and @rivet-dev/workflows, and add a migration note from rivetkit/workflow (actor({ run: workflow(...) })). History encoding is unchanged (per the README and tests/compat/upgrade.test.ts).
• Steps: replace the untyped inline ctx.try block with examples/docs/actors-workflows/try-block.ts, a typed workflow({...}) plus setup registry.

pnpm check-types passes; biome check examples/docs is clean.

[the-company-company]

✅ No issues found

_{Reviewed commit 873fc0b.}