DPE-2752 Add Discourse documentation + update metadata.yaml (#81)

## Issue

* documentation on Discourse was missing
* metadata.yaml was outdated (missing lings, outdated year, wrong
summary, etc)

## Solution

We are preparing the charm for release to stable and need to
generate all the necessary documentation/links on
https://charmhub.io/mysql-router
* update metadata.yaml to match mysql-router-k8s concepts
* add Discourse documentation Overview page

Author: taurus-forever