_content/doc: update install-source.html with changes from main repo
_content/doc: update install-source.html with changes from main repo

Copied from Go ff0e93ea3, deleted there in CL 291709.

Fixes golang/go#40496.
Fixes golang/go#41861.

Change-Id: Idb04706ea46b97be69fbeb98637c7ac2e4b54729
Reviewed-on: https://go-review.googlesource.com/c/website/+/291692
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website