Add visually hidden class and apply to h2 in search results
Changed h2 group headers to h3s
Add roles to main and content
Fix API material icon placement
Image alt additions where needed
Add lang attribute to html tag
Add aria-label on search input
All the docs related files (docs-app, doc-gen, content, etc)
are now to be found inside the `/aio` folder.
The related gulp tasks have been moved from the top level
gulp file to a new one inside the `/aio` folder.
The structure of the `/aio` folder now looks like:
```
/aio/
build/ # gulp tasks
content/ #MARKDOWN FILES for devguides, cheatsheet, etc
devguides/
cheatsheets/
transforms/ #dgeni packages, templates, etc
src/
app/
assets/
content/ #HTML + JSON build artifacts produced by dgeni from /aio/content.
#This dir is .gitignored-ed
e2e/ #protractor tests for the doc viewer app
node_modules/ #dependencies for both the doc viewer builds and the dgeni stuff
#This dir is .gitignored-ed
gulpfile.js #Tasks for generating docs and building & deploying the doc viewer
```
Closes#14361
