docs: add renovate logs link to dependency dashboards (#15)

The `dependencyDashboardFooter` option defaults to `null` so we don't
loose any information

Since it's part of the template, we can not link to the logs of the
repository, but this could be overridden in each repositories config if
needed.


https://docs.renovatebot.com/configuration-options/#dependencydashboardfooter

default.json

@@ -18,6 +18,7 @@
   "dependencyDashboardTitle": "Dependencies Dashboard (Renovate Bot)",
   "dependencyDashboardHeader": "Uses https://github.com/bettermarks/renovate-config#default  \nPR that have automerge enabled [might be approved automatically](https://github.com/bettermarks/approve-dependency-pr#readme)",
   "dependencyDashboardOSVVulnerabilitySummary": "none",
+  "dependencyDashboardFooter": "The renovate logs can be found at <https://developer.mend.io/github/bettermarks>",
   "osvVulnerabilityAlerts": false,
   "vulnerabilityAlerts": {
     "labels": ["security"],