Link to the observability part of the charming tutorial from our docs

After some conversations with @adhityaravi, we came up with the following idea:

We should link to the observability part of the charming tutorial from our docs; making sure people get a reference of a practical way of **doing** something with the observability stack, rather than just setting it up and being done with it.

https://documentation.ubuntu.com/ops/latest/tutorial/from-zero-to-hero-write-your-first-kubernetes-charm/observe-your-charm-with-cos-lite/#