⚙️ FEATURE-#184: Add NotifyDiscord provider and rename Notify.send to hook_status_task

[FernandoCelmer]

Description

• dotflow/abc/notify.py — Rename send() to hook_status_task()
• dotflow/providers/notify_discord.py — New Discord webhook notification provider
• dotflow/providers/notify_telegram.py — Refactored with show_result, _build_message, error details
• dotflow/providers/notify_default.py — Renamed method
• dotflow/providers/__init__.py — Export NotifyDiscord
• dotflow/core/task.py — Updated caller to hook_status_task
• Tests for NotifyDefault, NotifyTelegram, NotifyDiscord
• Docs updated across README, tutorials, integrations, custom-providers, mkdocs nav

Motivation and Context

Closes #184. Adds Discord webhook support following the same pattern as NotifyTelegram. The send() method was renamed to hook_status_task() to better describe when it is called.

Types of changes

• Bug fix (change that fixes an issue)
• 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