feat(A11y): Add inclusive design principles

What does this MR do?

I've created this MR to start a conversation. I know this current proposal is not right, but I'm confident together we can create something better. My end hope is 'accessibility' is defined and available, and the definition helps us design a GitLab that allows everyone to contribute.

All of the content is lifted from @jeldergl 's fantastic Inputs and outputs presentation TODO: add video link when available.

Preview the update

Note: it looks like Definition Lists are not supported in our markdown. These can be changed to something more suitable if we don't intend to support them.

Problem

As a designer I want to know what our definition of accessibility is, and principles I can use to design GitLab to be accessible.

Does this MR meet the acceptance criteria?

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  • For example, if this MR is focused on usage guidelines, addressing accessibility challenges could be added separately with a different MR
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