This PR addresses https://github.com/a11yproject/a11yproject.com/issues/669
It has minor copy tweaks to standardize the lede paragraph, some slight tone shifting, and adds more additional reading content.
I wasn't able to preview this locally, there's some strange bug where some content is repeating itself locally on the index page, only with the capitalization removed (see https://github.com/a11yproject/a11yproject.com/pull/696), and the posts themselves aren't updating. Throwing away the local build and rebuilding it didn't seem to work, either.
Another thing: While "A primer to vestibular disorders" follows the "A primer to X" convention we're using, it should be "A primer on/for/about vestibular disorders". I'm hesitant to update the title without having some sort of redirect in place, though.