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

[Meng-Heng]

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 is a rare case, but does reflect to the important of our help documentation.

Reproduce the bug

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Expected behavior

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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
• Other - give details at bottom of form

Keyman version

19.0

Operating system

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Device

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Target application

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Browser

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Keyboard name

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Keyboard version

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Language name

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Additional context

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