Document `documentation-assets`

- Document all projects that depend on these assets
- [Dyalog/documentation](https://github.com/Dyalog/documentation)
- [Dyalog/OpenAPI](https://github.com/Dyalog/OpenAPI)
- [Dyalog/documentation-guidelines](https://github.com/Dyalog/documentation-guidelines)
- Document *HOW* to use these assets. This may change after tools are added. Something like this:
```yml
site_name: PROJECT NAME
repo_url: https://github.com/Dyalog/REPOURL
copyright: Made with Material for MkDocs
Copyright © 1982-$CURRENT_YEAR Dyalog Limited ($BUILD_DATE $GIT_INFO).
Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution 4.0 International licence.

theme:
logo: documentation-assets/images/dyalog-logo_white.svg
features:
- navigation.instant
- navigation.tracking
- navigation.path
- navigation.indexes
- navigation.sections
name: material
font:
text: Be Vietnam Pro
custom_dir: docs/documentation-assets/overrides # for favicon

extra_css:
- docs/documentation-assets/css/main.css
- docs/documentation-assets/css/extra.css
extra_javascript:
- javascripts/mathjax.js
- https://unpkg.com/mathjax@3/es5/tex-mml-chtml.js

plugins:
- privacy
- search
- macros
- monorepo
- caption:
table:
enable: true
start_index: 1
increment_index: 1
position: bottom
reference_text: 'Table {index}'
caption_prefix: 'Table {index}:'
- minify:
minify_html: true

extra:
version:
provider: mike
generator: false

markdown_extensions:
- admonition
- pymdownx.details
- pymdownx.keys
- pymdownx.superfences
- pymdownx.arithmatex:
generic: true
- pymdownx.highlight:
pygments_lang_class: true
- attr_list
- abbr
- footnotes
- md_in_html
- markdown_tables_extended
- toc:
title: On this page

nav:
# Prefer a flat structure
```