Add a built-in theme witiko/diagrams for drawing different types of diagrams

[Witiko]

While PRs #521 and #529 implemented the major parts of MEP #514, it has not implemented the main motivating examples for the MEP: a new built-in theme for drawing diagrams.

A PR that closes this ticket and the MEP #514 should complete the following tasks:

1. Add a new built-in theme witiko/diagrams that will load the theme witiko/dot.
2. Only make version v1 of the theme witiko/diagrams load the theme witiko/dot and make the latest version:
   1. Use the syntax ``` dot {caption="Caption text"} instead of ``` dot Caption text.
   2. Support other diagram tools like Mermaid (see also #448), PlantUML, and TikZ.