docs: Add explanation of reloader types and terminal output

[commitWithTisha]

Summary

This PR adds documentation explaining the different reloader types that Flask uses and the terminal output users see when running in debug mode.

Problem

When running Flask with debug mode enabled, users see messages like:

• * Restarting with stat
• * Restarting with watchdog (windowsapi)
• * Restarting with inotify

These messages are never explained in the documentation, which can confuse users (especially beginners) who don't understand what they mean or why they appear.

Changes

docs/server.rst

• Added new "Understanding the Reloader" section explaining:
  • The two reloader backends (watchdog vs stat)
  • Why Flask chooses different backends automatically
  • What the platform-specific names mean (windowsapi, inotify, kqueue)
  • How to install watchdog for better performance
  • How to manually specify a reloader type if needed

docs/quickstart.rst

• Added a note in the Debug Mode section that references the new server.rst section
• Helps users immediately understand the reloader output they see in the quickstart example

Testing

• Verified reStructuredText syntax is correct
• Checked that internal documentation links work properly
• Confirmed the new content fits naturally with existing documentation structure

Related

The installation docs already mention watchdog as an optional dependency, but don't explain what it does or when it's used. This PR bridges that gap.