docs: add documentation for ngProjectAs

[jotatoledo]

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

The docs dont mention this feature anywhere afaik.

Expected behavior

Describe the use of it.

[alexzuza]

Dup of #21762

[jotatoledo]

@alexzuza my bad, should had done a quick search before opening. Thanks!

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