View-page URL should use `blob` not `tree`

[chalin]

See https://github.com/google/docsy/blob/bd7f023e8894e68f39b86f0482efe43f25d9e318/layouts/_partials/page-meta-links.html#L31

While we're there, maybe also consider adding a UTM source, similar label, to the URL so that we can identify that it comes from Docsy's View Source link. This would make it easier to filter out during link checking.