Support watching jsonnet via server

[malcolmholmes]

Currently, the grr serve command will only watch its resourcePath for changes. This works fine for a directory full of resources, such as JSON or YAML files. But for Jsonnet, it doesn't.

This PR adds a simple change to allow a user to provide an additional argument, that is, a "watchDir" path, so this command:

grr serve -w examples/grr.jsonnet examples

will allow a user to edit examples/test-dashboard.json, which is included in grr.jsonnet, and see that change show immediately in Grafana.

This PR also updates the docs for the Grizzly server, including this change in them.

[Duologic]

The additional path is a good quick fix to support jsonnet initially. If it can support multiple directories/files, we could do something like this:

grr serve -w examples/grr.jsonnet $(tk tool imports examples/grr.jsonnet)