doc: update "go get" HTTPS answer to mention .netrc

The existing text makes it seem like there's no way
to use GitHub over HTTPS. There is. Explain that.

Also, the existing text suggests explicit checkout into $GOPATH,
which is not going to work in the new module world.
Drop that alternative.

Also, the existing text uses pushInsteadOf instead of insteadOf,
which would have the effect of being able to push to a private
repo but not clone it in the first place. That seems not helpful,
so suggest insteadOf instead.

Fixes #18927.

Change-Id: Ic358b66f88064b53067d174a2a1591ac8bf96c88
Reviewed-on: https://go-review.googlesource.com/107775


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>