docs: add keybindings reference and link from README

[Redooyyy]

Summary

Adds a dedicated SHORTCUT.md file documenting all keyboard shortcuts
for the Caelestia shell config, and links it from the README for easy access.

Changes

• Added SHORTCUT.md with all shortcuts organized by category
• Linked SHORTCUT.md from README under the Shortcuts/IPC section

Notes

• Shortcuts were resolved from both keybinds.conf and variables.conf
• Categories: Launcher, Apps, Workspaces, Window Management, Panels,
  Screenshots, Volume, Media, Clipboard, Session, and Developer shortcuts

[Kafkacodes]

The shortcuts documentation is very helpful, since many users have been asking for a keyboard shortcut reference. Thanks for the contribution.

[Kafkacodes]

CI is currently failing on the QML formatting check and the branch also needs conflict resolution before merge.
Fix the formatting.