Update the Theme Creation page to include information about the Hidden key

[philsward]

See backdrop/backdrop-issues#5321 for context of this issue

On the theme creation document page under the Contents section, there needs to be a new "key" for the hidden flag.

I would place this section between: base theme & theme settings

Copy & Paste (or edit for use)

Visibility Setting

hidden = false

The visibility key setting allows the theme to be shown or hidden from the administrative view list page. This will help by removing themes that don't need to be exposed or changed through the GUI. This is a Boolean setting which means options for this are either true or false. This is an optional setting, however if you are creating a contributed theme, it is recommended to always make sure this is added to the .info file for best consistency. The default for this setting is false if the key is missing or true if the key is present with no value.

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See backdrop/backdrop-issues#5329 for details on what the actual "default" workflow is supposed to be... It's either an odd workflow by design or an incorrect workflow that needs fixed.

Update 2021-10-28: Some eyes were able to look at #5329 and it is agreed that a null value should act the same as no value, therefore the default statement should read something like: "The default for this setting is false if the key is missing or the value is anything other than true."

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Rhetorical question: Are there any other areas where this needs to be documented and hasn't been?

[philsward]

It appears this is documented in the modules portion of docs: https://docs.backdropcms.org/creating-modules#info-files (search for hidden)