Add a11y HTML tips

### Description 
I want to share to all developer some tips relating to accessibility, based on my experience, to enable some "checklist of release" and get full support for all user on the web.

### Doc Domain
- [x] Html
- [ ] CSS
- [ ] JavaScript
- [ ] Bootstrap

#### Are you contributing under any open-source program ?
Hacktoberfest, Gatsby, Next, Eleventy
