Add Mermaid.js use for diagrams

[flfagundes]

Amazing work team, thanks a lot. Incredible tool.
One suggestion, is created the possibility to use Mermaid.js to build diagrams in Markdown on the documents, could be very useful.

Thanks one more time, really impersonated.

[BykhovDenis]

Done

[MichaelUray]

To add on the earlier "Done": in a document, type / and choose "Diagram" to insert a Mermaid block, and the diagram renders under the source. Pasting Markdown that contains a ```mermaid fenced block also turns it into a diagram (#8620, #10714), and diagrams can be opened in fullscreen (#10477).

Note: this repository is frozen; development continues in Platform-Collective/platform (see the README).