Adding all code components for a new 'markdown' display_template

[SummittDweller]

Pull request

Proposed changes

In my personal project, https://github.com/SummittDweller/1926-miriam-mcfate-journal, I saw the need for a display_template that would simply render a Markdown document similar to what CB CSV was already doing with PDFs and other types. I think this delivers on that requirement.

You can see my Azure static web app deployment of these additions at https://thankful-sea-06fcb2e0f.4.azurestaticapps.net/. There are no images here yet and no changes other than the addition of Markdown content, but you can see what I'm up to in the Browse page especially.

Types of changes

• New feature (non-breaking change which adds functionality).
• Enhancement (non-breaking change which enhances functionality)
• Bug Fix (non-breaking change which fixes an issue).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[evanwill]

Thanks @SummittDweller , this looks impressively complete! Give us a bit to take a look.

[Digital-Grinnell]

You are very welcome. Let me know if there's more that I can do to assist regarding this PR. One thing I thought to add, after it was submitted... Not sure if there's already documentation around the concept, but if not, it might be worth mentioning that Markdown, like transcripts, must be stored locally in the project (or just appear as text) without the option of external references like other content types have. After last evening I have a much better appreciation for why one would NOT want to implement external references for such content!