Support of Mermaid charts for Markdown files

[pylapp]

Feature Name

Support of Mermaid charts in HTML pages

Description

Today, markdown files can contain charts so as to explain things or display workflows for example.
Instead of using images, charts can be designed simply with text content thanks to tools like Mermaid.

In addition, this tool is supported in GitHub side and helps people to draw charts simply: the content is written in text, placed in the markdown, and then rendered using Mermaid.js.

You can find the announcement of GitHub in this page.

Motivation

Having charts nicely rendered in the generated DocC file will help users to read them and unserstand aprts of the project for example.

For example, given the following README file, the rendering of the HTML generated page using version 1.4.2 of swift-docc is this one.

If the file is viewed through GitHub web site, the rendering is done with automatically zooms and other features.

Importance

This is a new feature which will help users and prevent them to go to the project repository through GitHub to get the documentation. The need can be filled with images of the charts must maanging images is not interesting.

Alternatives Considered

None, maybe images of charts but we don't want that.

[heckj]

@pylapp I think this likely resides with swift-docc-render, where there's an existing issue requesting this support: swiftlang/swift-docc-render#413. This repo is the face of docc, but the actual rendering of content happens in the other repo, and the source of this repo is more focused on data transformation for symbol graphs into docc archives that are in turn rendered by https://github.com/swiftlang/swift-docc-render/