[Shortcut] Add an option in preferences to open keybindings.json

[YalandHong]

Describe your feature request

Add an option entry in preferences to open keybindings.json

What problem does this feature solve? [optional]

I think people are too lazy to check KEYBINDINGS.md in your documents. We should put it into another place where they can easily find.

I have seen a huge amount of issues where people complain about shortcut conflicts and customization:

1. Make keybindings changeable #2671
2. Right Alt/"Alt gr" doesn't work on macOS #2645
3. I have conflicts when using shortcut keys and I have some suggestions #2599
4. Add a section to set hotkeys & the Obsidian Shortcuts #2587
5. Same Shortcut Issue #2415
6. Several issues with shortcuts in Linux Mint #2370
7. Custom Shortcuts #2698

Additional context [optional]

I think it would not be difficult to implement this feature. It only requires MarkText to check the existence of keybindings.json, create it then open it in your default text editor, just like what VSCode does with its settings.json.

[YalandHong]

Merge into #2801