Add link to upgrade documentation to log message

Add link to upgrade documentation to log message

When there's a schema version mismatch after an upgrade, the following
message is logged:

```
Schema version 'x.y.z' does not match the required version 'x.y.z' (or
newer)! Please check the upgrade documentation.
```

After reading this log message, users have to search for the upgrade
section in the docs in order to perform the necessary steps.

I added the URL for the relevant sections to the log message, so users
can simply click/copy it.