docs: asynchronous actions

[ogorzalka]

New page async-actions.md for Async Actions (pollora/hook 1.4 + framework phases 3 and 4), to merge with the framework release that ships them.

• Opens with the four rules (at least once, no order, variable delay, class over closure), as the design document asks.
• ->async(), #[Async] (several hooks, class level, ignored declarations), pollora:make:action --async.
• Options table, hook arguments and what travels, AsyncContext, capture(), when().
• Drivers, auto and the queue opt-in (HOOKS_ASYNC_CONNECTION), config/hooks.php.
• Something must run the queue: Pollora sets DISABLE_WP_CRON, so WP-Cron and Action Scheduler need a system cron (or a worker with wordpress.use_laravel_scheduler); the queue needs a worker.
• Retries, pollora/async/failed, closures, Async::fake(), pollora:async:list, the doctor check, use without the framework.

Also: links from README, hooks.md and the doctor table in dashboard.md. pollora.dev needs a sync_file line and a link rule in scripts/sync-docs.sh for the new page.

Framework PRs: Pollora/framework#430 (doctor), #431 (--async), and the pollora:async:list PR that follows #430.