WIP: A11ydoc - copyedits

[naomiblack]

This is still too long... I'm concerned that developers who need to learn about accessibility (but maybe don't share our passion for the topic) won't have the patience to read a 30-page document.

@AlmeroSteyn -- I did a quick copyedit to make the tone a little more dry so it matches the other cookbooks, and to try to keep it more tightly focused to just the essentials. I cut the length by about a third, but I'm worried that it's still quite long.

@wardbell have a look and see if you can tighten it up further?

[googlebot]

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We found a Contributor License Agreement for you (the sender of this pull request) and all commit authors, but as best as we can tell these commits were authored by someone else. If that's the case, please add them to this pull request and have them confirm that they're okay with these commits being contributed to Google. If we're mistaken and you did author these commits, just reply here to confirm.

[AlmeroSteyn]

This PR is outdated and should be closed for preference of #1625