Move autoprefixer browser compatibility info from build.md to browser-support.md

[jenniferfell]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[ ] Feature request
[X ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question
[ ] Other... Please describe:

Current behavior

https://next.angular.io/guide/build#configuring-browser-compatibility
contains information about using Autoprefixer to ensure compatibility with different browser and browser versions.

Expected behavior

This content makes more sense in Browser Support
https://next.angular.io/guide/browser-support

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

Request from @IgorMinar during a doc review.
Easier to find if all browser-specific information is in one location.

Environment

Angular version: X.Y.Z


Browser:
- [ ] Chrome (desktop) version XX
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
For Tooling issues:
- Node version: XX  
- Platform:  

Others:

[jenniferfell]

@IgorMinar Do I recall correctly that you want us to move this content? Thx

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