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.

[Redooyyy]

All conflicts resolved!! I guess it's ready to merge!!

[soramanew]

We are currently working on the website, so hopefully we can put this on the website instead of in the README

[Kafkacodes]

@soramanew are we expecting caelestia own wiki?

[sinder38]

Wanted to open a PR about this but it feels more appropriate to review this instead.