docs: document telegram proxy config

[yzlu0917]

Summary

• add Telegram proxy options to config.example.toml
• document HTTP/SOCKS5 proxy setup in docs/telegram.md
• show optional proxy auth fields alongside the Telegram platform config

Why

Telegram proxy support already exists in the platform implementation, but the example
config and setup guide did not show how to enable it. This makes the feature easier
to discover and configure.

Validation

• reviewed the Telegram platform implementation to match actual option names
• checked the modified files with git diff --check

[chenhg5]

LGTM. Documentation for existing proxy feature.

Review summary:

• ✅ Documents HTTP/SOCKS5 proxy options for Telegram
• ✅ Adds to config.example.toml
• ✅ CI passes

Makes the existing proxy feature discoverable.

[chenhg5]

⚠️ Merge Conflict

This PR has merge conflicts with the main branch. Please resolve the conflicts:

git fetch origin
git checkout your-branch
git merge origin/main
# Resolve conflicts, then:
git push