* emails: use an https image for the logo

Some mail clients do not like http images.

* emails: use an https:// link for "getting started"

* email: doc.grafana.com for Getting Started link

review suggestions from https://github.com/grafana/grafana/pull/25957

Co-authored-by: Sofia Papagiannaki <papagian@users.noreply.github.com>

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>
Co-authored-by: Sofia Papagiannaki <papagian@users.noreply.github.com>