docs(solidstart): add cron jobs guide using Nitro Tasks API (#1534)

* docs(solidstart): add cron jobs guide using Nitro Tasks API

Documents how to set up scheduled background tasks in SolidStart
using Nitro's Tasks API. Covers configuration in app.config.ts,
task file structure in tasks/ directory, and manual triggering
via the Nitro task endpoint.

Closes #964

* docs: address copilot review on cron jobs guide

- Add caution block warning that /_nitro/tasks/* is publicly accessible
  in production (node-server, bun, deno-server) and should be protected
- Add curl example with explicit GET method for task triggering

* docs(solidstart): address review on background tasks guide

Rework the SolidStart tasks guide per maintainer review:
- rename guide to Background tasks (Nitro calls them tasks, not all are cron)
- expand intro and link Nitro platform-support docs instead of hardcoding presets
- mark tasks as experimental and split scheduling into its own section
- use a practical cleanup-sessions example with package-install-dev
- remove the production caution: /_nitro/tasks is a dev-server-only endpoint
  (registered solely in the _nitro-dev preset via dev-tasks.ts), not exposed
  by node-server/bun/deno-server presets

* ci: apply automated fixes

* format

* Remove unused import

---------

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Amir Hossein Hashemi <87268103+amirhhashemi@users.noreply.github.com>