Add guide for migrating a Celery task queue to Temporal

Add docs/guides/celery-to-workflow.mdx, a Python guide that walks
through converting a Celery application to Temporal one piece at a
time: mapping Celery concepts to Temporal primitives, turning a task
into an Activity, orchestrating it with a Workflow, running a Worker,
replacing .delay() with client.start_workflow(), migrating task
retries to a Retry Policy, replacing Celery Beat with a Temporal
Schedule, and translating Canvas workflows.

Link the page from the Guides sidebar and add a card to the Guides
grid under a new "Migration" tag.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: brianmacdonald-temporal