Add support for mermaid diagrams in charm/lib docs

[sed-i]

As the ecosystem grows it becomes valuable to convey information in diagrams.

At the moment, the observability team uses mermaid to outline charm deployment scenarios, for example:

• https://github.com/canonical/prometheus-scrape-config-k8s-operator/blob/main/INTEGRATING.md
• https://github.com/canonical/grafana-agent-k8s-operator/blob/main/INTEGRATING.md#deployment-scenarios

Other examples:

• https://discourse.charmhub.io/t/a-charms-life/5938

Mermaid is plain-text so easily diffed/bisected/etc.

Needing to upload .png to discourse means we need to keep the source somewhere, which means overlap between github .md docs and discourse docs, which we'd need to merge manually both ways every time we need to update.

We would be able to better use discourse and reduce overlap between github *.md and discourse posts if charmhub and discourse supported mermaid.

[simskij]

Pinging @evildmp as he might have thoughts on this.