Split plugin and theme templates into smaller modular templates

[argenos]

• The jinja templates are divided into smaller files, which means they can be used to update smaller sections of other templates (e.g. for Make manual edits of Author/People pages less error-prone #166)
• This PR only partially addresses Make jinja templates modular: #156

Checklist

• I have renamed all attached images with descriptive file names (or I did not include any images)
• Before creating a new note, I searched the vault (or I only edited existing notes)
• (Optional) In case I created a new note in the folder 04 - Guides, Workflows, & Courses, I added a link to the new note(s) in one of the for {group X} overviews (listed here).

[claremacrae]

This is brilliant - I had no idea that dividing the templates up in to chunks would be possible like this.

I tested it by running this:

./update-releases.py --all --overwrite

And confirming that the existing themes, plugins and authors were generated as expected.