Add Wikimedia-style templates

[ozxybox]

It would be fantastic if we could get Wikimedia-style templates.

We should be able to make a HTML template in the Wiki repo, store it under templates/ or something like that, and then refer to it from the markdown.

An example from Wikipedia would be their "under construction" banner to tell users that a page is a work in progress.
For us, we would put an HTML file with the banner into templates/UnderConstruction.html. Then from a markdown file, writing {{UnderConstruction}} should substitute the tag with the contents of the HTML file.

Reference pages
https://en.wikipedia.org/wiki/Help:Template
https://commons.wikimedia.org/wiki/Commons:Templates
https://gohugo.io/templates/introduction/

Note: should we reuse the existing template substitution syntax for this? https://github.com/StrataSource/strata-wiki/blob/5cda791c8446246a985567609256eba14235b482/src/exporter/template.ts#L49C1-L53C10