[content idea] a11y screen reader and voice control tips for specific HTML features

While testing AMP on iOS, we came across some browser-specific a11y issues, for example with ``, it can't be focused when using VoiceOver.

While this is not an issue with AMP per se, I think it would be valuable to mention somewhere (either in `` docs or a separate article) that input types like this should be avoided to prevent a11y issues.

I found a great website that tests and lists all of these issues in different browsers - might be worth investigating what other things have poor cross-browser support currently: https://a11ysupport.io/tests/

Review the existing amp-form documentation and the a11ysupport.io tests for browser-specific issues with HTML features such as input type="month". Decide whether the guidance belongs in amp-form docs or a separate article, then document actionable VoiceOver and voice-control recommendations with the relevant cross-browser limitations.

Written by the indexing model from the issue text.