_content/ref/mod: clarify that go.mod files are not required for dependencies
_content/ref/mod: clarify that go.mod files are not required for dependencies

We already mentioned this fact elsewhere in the document, but the
"go.mod files" section is a likely starting point for someone trying
to figure out what happens when the file is missing. This adds some
prose and links there to point the reader to the appropriate sections.

Noticed via https://stackoverflow.com/q/67030123.

Change-Id: Ia7fe50a8b97b8cc1320778c318e5b3b0694d7c05
Reviewed-on: https://go-review.googlesource.com/c/website/+/331949
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

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