⚙️ FEATURE-#103: Add async/await support for actions

[FernandoCelmer]

Description

• dotflow/core/action.py: Detect async functions with asyncio.iscoroutinefunction(), run with asyncio.run(), handle existing event loops (FastAPI/Django) via thread pool fallback
• docs_src/async/async_action.py: Usage example with async and sync actions
• docs/nav/tutorial/async-actions.md: Tutorial page with examples, mixing sync/async, retry, when to use
• mkdocs.yml: Added async-actions to nav

Motivation and Context

Most workflows are I/O-bound (HTTP calls, DB queries, file operations). Async support enables concurrent I/O within actions without changing the workflow API. Works in all environments: scripts, Lambda, FastAPI, Django async.

Closes #103

Types of changes

• New feature (change which adds functionality)
• Documentation

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the CHANGELOG
• I have updated the documentation accordingly