replace “sidebar” with “panel” in README.md

[Strangehill]

Begins to address issue #57546

What?

Replacing the word “Sidebar” with a “Panel”. The former term is a visually oriented descriptor, and the replacement doesn’t have this. There are other benefits, listed in the originating issue such as the ease of translation.

Why?

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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[github-actions]

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[github-actions]

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[mirka]

Looks good, thank you!