docs: rewrite Celery migration table notes for specificity

[pratyush618]

The "Notes" column on the Celery → taskito concept-mapping table was too thin to actually help someone migrating. It mostly restated the obvious ("Same name, similar API", "Same decorator pattern", "Similar CLI") without telling the reader what specifically changes when they switch.

This PR rewrites every Notes cell to surface the practical delta — what kwargs carry over, what the return type becomes, what flags are new in taskito.

Before vs after

Celery	taskito	Before	After
`Celery()` app	`Queue()`	No broker URL needed	`Queue(db_path=...)` replaces `Celery(broker=..., backend=...)` — no broker URL, no result-backend URL, no `app.conf` setup.
`@app.task`	`@queue.task()`	Same decorator pattern	Same decorator shape; `max_retries`, `rate_limit`, `bind=True`, `name=` carry over with identical names.
`.apply_async()`	`.apply_async()`	Same name, similar API	Same kwargs (`countdown`, `eta`, `priority`, `queue`); taskito adds `delay`, `unique_key`, `expires`, `depends_on`.
`.delay()`	`.delay()`	Identical	Drop-in. Returns `JobResult` instead of `AsyncResult`.
`AsyncResult`	`JobResult`	`.result()` instead of `.get()`	`.result(timeout=...)` replaces `.get(timeout=...)`; async variant `await job.aresult()`.
Canvas	`chain`, `group`, `chord`	Same names, same concepts	API-compatible. taskito adds a DAG `Workflow` builder with conditions, gates, and fan-in.
`celery beat`	`@queue.periodic()`	Built-in, no separate process	No separate beat daemon. Cron expressions support seconds (6 fields) and live in the worker process.
Result backend	Built-in (SQLite)	No configuration needed	Results store in the same SQLite file as jobs. Per-job `result_ttl_ms` controls retention.
Broker	Not needed	SQLite handles everything	A single SQLite (or Postgres) connection replaces the broker. WAL mode allows concurrent reads.
`celery worker`	`taskito worker`	Similar CLI	`--concurrency`, `--queues`, `--loglevel` carry over. taskito adds `--pool prefork` for CPU parallelism.
`celery inspect`	`taskito info`	Similar CLI	Covers `inspect active/scheduled/registered`. `taskito dashboard` is the live UI version.

Conventions used

• Lead with the concrete API delta where applicable (`X replaces Y` / `Drop-in. Returns Z`).
• Mention taskito-only additions only when proximate (e.g. `--pool prefork` on `worker`, `unique_key` on `apply_async`) — keeps each row scannable.
• Backtick everything that is code.
• Tone stays casual; no row grows past two sentences.

Test plan

• `pnpm --dir docs build` — clean build
• CI: `Deploy Docs` job
• Visual smoke: open `/docs/guides/operations/migration` and confirm the table reflows correctly with the wider note cells in dark + light mode.