Improve visualisation of schema

[stufraser1]

It would be nice to come up with some more interactive way to navigate the UML diagrams shown at https://gfdrr.github.io/rdl-doc/. I can read UML, but still struggle without a legend for elements and field names. I wonder if we can create something more interactive and easier to navigate/understand

e.g. https://yuml.me/diagram/scruffy/class/samples

[matamadio]

More plugins for generating flowcharts in md:

https://medium.com/technical-writing-is-easy/diagrams-in-documentation-markdown-guide-4e78419e8d2f

https://support.typora.io/Draw-Diagrams-With-Markdown/

[pzwsk]

Moving to doc repo.