docs: add comprehensive user manual for project integrations (Telegram, Discord, Slack, Mattermost, Zulip)

[maruf-pfc]

This PR significantly expands the project integrations documentation (using-leantime/integrations.md) into a comprehensive, step-by-step User Manual covering all messaging and webhook integrations supported natively by Leantime.

What's Included:

• Telegram:
  • Step-by-step bot creation using @BotFather.
  • Documentation of the automatic Chat ID & Topic ID detection flow (sending a message to the bot and clicking Save).
  • Manual Chat ID and Topic ID configuration for Telegram Forum Supergroups.
  • Notification format overview (title, status badge, priority, assignee, due date, deep link).
• Discord:
  • Webhook generation in Discord server settings.
  • Multi-channel support using up to 3 separate webhook URLs per project.
  • Rich embed card payload details.
• Slack:
  • Incoming Webhooks app configuration and channel targeting.
• Mattermost:
  • Incoming webhook setup for self-hosted and cloud instances.
• Zulip:
  • Bot credentials setup (Bot Email, API Key), Base URL, stream, and topic targeting.
• Notification Triggers:
  • Explains what triggers notifications (To-Dos, Milestones, Ideas, Research Boards, Comments, Files).
• Project Muting & Notification Controls:
  • Explains how project muting works and the Muted by X team members indicator.
• Troubleshooting & Security:
  • Common pitfalls including OutboundUrlGuard / SSRF private IP rules, Telegram chat_not_found, and LEAN_APP_URL link resolution.

Navigation & Asset Improvements:

• Official Vector SVGs: Added lightweight, crisp vector SVGs for Telegram, Discord, Slack, Mattermost, and Zulip.
• Sidebar Cleanliness: Added a targeted .sidebar-nav img { display: none !important; } CSS rule in index.html so platform icons appear cleanly next to headings on the page without cluttering Docsify's sidebar menu.
• Navigation Links: Added [Project Integrations](using-leantime/integrations.md) to _sidebar.md under Configuration and cross-referenced it in installation/configuration.md.

Verification

• Tested locally using Docsify (docsify serve).
• Verified that all internal links, sidebar navigation, and SVG assets render with HTTP 200 OK.