Avoid the term 'sidebar' in the Handbook

[afercia]

Description

In #49613 and #49614 the term 'sidebar' was removed from most of the user-facing text / labels / names. In the same way, it should be removed from the Handbook and replaces with 'panel'.

The main points for this proposed change are:

• The whole concept of 'sidebar' is extraneous to blind screen reader users. The term 'sidebar' is a positional reference, as it refers to something that is placed on a 'side'. As a blind user, it doesn't help me to know whether a section of the user interface is visually placed on the side, top, bottom or in any other spot of the screen.
• Ideally, UI sections should be named based on their functionality rather than on their visual aspect. Same applies for components.
• On small screens, most of these UI sections aren't 'sidebars' any longer. They're more full-width overlay panels so that the term 'sidebar' isn't accurate in this case.
• When translated, the term 'sidebar' is generally way longer than in English and that doesn't help readability.

A more appropriate, generic, term that refers to functionality rather than appearance would be 'panel'. The term 'panel' is widely used in software.

It is worth pointing out that right now the term 'panel' is used a little inconsistently in the Handbook. See for example the following two examples where the Settings 'sidebar' is also called 'panel' and the single group of settings within it are called 'panels' as well.

From https://developer.wordpress.org/block-editor/

The elements highlighted in the figure are:
...
Settings Sidebar: A sidebar panel for configuring a block’s settings (among other things)

From https://developer.wordpress.org/block-editor/getting-started/tutorial/

Save the file and select the block in the Editor. You will now see both Color and Typography panels in the Settings Sidebar.

Step-by-step reproduction instructions

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Screenshots, screen recording, code snippet

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Environment info

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Please confirm that you have searched existing issues in the repo.

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Please confirm that you have tested with all plugins deactivated except Gutenberg.

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