Add accessibility documentation

[HedwigAR]

Here's the first draft of new accessibility documentation. The goal of this is to explain accessibility in general and give some pointers for people how to make their apps more accessible. It's very much meant as a starting point that later documentation can build off of.
It currently still needs some images of certain settings but the text should be all there already.

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✅ ConnorLand
❌ HedwigJDoets
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[ConnorLand]

What does this mean "instead of using Click here? I think this example needs to be a bit more spelled out.

[HedwigAR]

This refers to a situation where you have a link in a block of text. For example, on a museum website you could have something like: [Click here](url) to learn more about fossils
This would not comply with accessibility best practice. Instead people should do something more like: To learn more about fossils, go to our [fossil collection](url)

I'll add a mendix specific example

[ConnorLand]

Edits on this doc are mostly done. The work remaining is to:

1. Make a new parent doc in Modeling titled something related to best practices
2. Move this doc and 2 others into the new category
3. Fix broken links
4. Set up aliases in the moved docs' metadata

I will finish that work when I get back and this should merge either Nov 7 or 10.