Sharing content between markdown files

In order to link the same content in multiple markdown files, you need to keep the actual markdown content in a separate file without frontmatter. This can be done using a custom plugin that comes with the Unidata jekyll theme. For example, information about how to handle headings using markdown is duplicated between the Markdown Overview and the Style Guide pages. The way this works is that the content about how to use headings in stored in the file pages/unidata/sharing_content.md, which looks like this:

## Headings

All headings should start off as level 2.
The `title` set in the front matter will be displayed at the top of the rendered html page and uses the level 1 heading.

Then, the includefiles plugin is used to insert the content wherever it should be used. For example, the beginning of the style guide markdown file (located at pages/unidata/style_guide.md) looks like:

---
title: Style guide
last_updated: 2021-04-01
toc: false
sidebar: unidata_sidebar
permalink: style.html
---


{% capture rmd %}{% includefile pages/unidata/shared_content/headings.inc %}{% endcapture %}
{{ rmd | liquify | markdownify }}


## Technical jargon

...

and the relevant part of the Markdown Overview markdown file (located at pages/netcdfJava/developer/DocGuide.md) looks like:

---
title: Markdown Overview
last_updated: 2021-04-01
toc: true
sidebar: unidata_sidebar
permalink: overview.html
---

## General Kramdown Syntax

...

Here are some highlights to get you going.

{% capture rmd %}{% includefile pages/unidata/shared_content/headings.inc %}{% endcapture %}
{{ rmd | liquify | markdownify }}

## links

General format is:
...

Note that in both cases, the content of the file is stored into a liquid variable called rmd using capture. Then, the variable is passed through a liquify filter, which will process any liquid templates contained within the markdown. After the liquify filter, the resulting text is passed through the markdownify filter, which turns the markdown into html.bv