docs(developer): define the characters in the Layout tab instead of On-Screen

### Describe the bug

1. https://help.keyman.com/developer/19.0/context/keyboard-editor#toc-on-screen-tab
2. https://help.keyman.com/developer/19.0/context/keyboard-editor#toc-layout-tab

While trying to see if there is a guide on how to use Layout and On-Screen in Keyman Developer, I only came across the information unrelated to the warning of defining characters in the On-Screen tab does not reflects onto the Layout tab. The information from the [community site](https://community.software.sil.org/t/installed-keyboard-is-not-working/11402/10) is a rare case, but does reflect to the important of our help documentation.

### Reproduce the bug

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### Expected behavior

_No response_

### Related issues

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### Keyman apps

- [ ] Keyman for Android
- [ ] Keyman for iPhone and iPad
- [ ] Keyman for Linux
- [ ] Keyman for macOS
- [ ] Keyman for Windows
- [ ] Keyman Developer
- [ ] KeymanWeb
- [x] Other - give details at bottom of form

### Keyman version

19.0

### Operating system

_No response_

### Device

_No response_

### Target application

_No response_

### Browser

_No response_

### Keyboard name

_No response_

### Keyboard version

_No response_

### Language name

_No response_

### Additional context

_No response_