Improve shortcut formatting in docs

[YuriSizov]

Followup to godotengine/godot-docs#3257.

Added a new [kbd] tag for docs, similar to <kbd> tag here on GitHub. It translates into :kbd:`` when generating RST-files. For built-in docs a style similar to [code] was chosen, but the font is one point smaller and has a different color. I think, if #35608 is merged, the [kbd] tag should be then made more visible with a darker background color.

Here is the worst example for built-in help:

I've decided not to add this tag in some cases, like the KEY_ constants in @GlobalScope. I don't think that it would help readability there and some of the key names are very descriptive or specific. Can be easily corrected though, now that we have a tag.

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If I'm understanding the purpose of the modules/mono/editor/bindings_generator.cpp file correctly, this new tag is also excluded from XML docs in C# API bindings as unsupported.

[YuriSizov]

Just in case anybody was wondering: the build was fine, the information here did not update for some reason.

[neikeq]

If I'm understanding the purpose of the modules/mono/editor/bindings_generator.cpp file correctly, this new tag is also excluded from XML docs in C# API bindings as unsupported.

Yes, changes look good. It will add the tag content as normal text.

[akien-mga]

Thanks!