Adds Mermaid to the docs configuration

[degville]

(cc @ernestl)

With this configuration change, we can now embed Mermaid diagrams within our documentation.

In MyST, diagrams are placed within ```{mermaid} ``` blocks, such as for the following timeline:

%%{init: {"theme":"forest"}}%%
timeline
    title Ubuntu recent releases

    2020 : 20.04 LTS (Focal Fossa) : 20.10 (Groovy Gorilla)
    2021 : 21.04 (Hirsute Hippo) : 21.10 (Impish Indri)   
    2022 : 22.04 LTS (Jammy Jellyfish) : 22.10 (Kinetic Kudu)
    2023 : 23.04 (Lunar Lobster) : 23.10 (Mantic Minotaur)
    2024 : 24.04 LTS (Noble Numbat) : 24.10 (Oracular Oriole)
    2025 : 25.04 (Plucky Puffin)

See https://canonical-starter-pack.readthedocs-hosted.com/stable/how-to/diagrams-as-code-mermaid for further details.