Update Markdown page with links (#642)

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* Add Markdown to glossary

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Co-authored-by: Brian Rose <brose@albany.edu>
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appendix/glossary.md

@@ -88,6 +88,9 @@ Linux
 local
 : Refering to something, such as a {term}`Git` repository, on your computer rather than a remote server.
 
+Markdown
+: A simple, human-readable text language used extensively in {term}`Jupyter Notebooks`, on {term}`GitHub` and other code repositories, and elsewhere on the Internet (e.g., forums and blogs). Markdown lets us write readable plain text that can rendered as nicely styled text on web pages and other media, with headings, links, and other formatting.
+
 merge
 : A specific type of commit that combines changes from two branches.
 

foundations/markdown.md

@@ -1,9 +1,11 @@
 # Formatted Text in the Notebook with Markdown
 
-```{note}
-This content is under construction!
-```
+{term}`Markdown` is a simple, human-readable text language used extensively in {term}`Jupyter Notebooks`, on {term}`GitHub` and other code repositories, and elsewhere on the Internet (e.g., forums and blogs). Markdown lets us write readable plain text that can rendered as nicely styled text on web pages and other media, with headings, links, and other formatting.
 
-This section will give a tutorial on formatting text with Markdown: the simple, human-readable text language used extensively in Jupyter notebooks, GitHub Discussions, and elsewhere.
+Project Pythia uses [MyST Markdown](https://mystmd.org/), which is an extension of [CommonMark](https://commonmark.org/), which is in turn an implementation of [Markdown](wiki:Markdown).
+In fact, the page you are reading right now was written in Markdown! See the [source on GitHub here](https://github.com/ProjectPythia/pythia-foundations/blob/main/foundations/markdown.md).
 
-We will show how this is useful both in notebooks and other places like GitHub Issues.
+Markdown was briefly mentioned in the previous section in the context of {term}`Jupyter Notebooks`. Here we provide a list of recommended resources for learning more.
+- To learn the basics: the [CommonMark docs](https://commonmark.org/help/), including their [10-minute interactive tutorial](https://commonmark.org/help/tutorial/)
+- To learn some of the MyST-specific syntax: the [Introduction to MyST Markdown page](https://jupyterbook.org/stable/authoring/mystmd/) within the Jupyter Book User Guide
+- To learn everything that you can do with MyST: the [Authoring section of the MyST guide](https://mystmd.org/guide/typography)