About content reuse
You can embed content from one file inside another using includes. Put the file containing content you want to reuse (e.g., mypage.html) inside the _includes/mydoc folder (replacing “mydoc” with your project’s name), and then use a tag like this:
{% include mydoc/mypage.html %}
With content in your _includes folder, you don’t add any frontmatter to these pages because they will be included on other pages already containing frontmatter.
Also, when you include a file, all of the file’s contents get included. You can’t specify that you only want a specific part of the file included. However, you can use parameters with includes. See Jekyll’s documentation for more information on that.
Re-using content across projects
When you want to re-use a topic across projects, store the content in the \includes folder (it can be in any project’s subfolder). Any folder that begins with an underscore (_) isn’t included in the site output.
Also be sure to put any images in the common_images folder. None of the assets in the common_images folder should be excluded in the configuration files. This means every project’s output will include the resources from the common_images folder.
However, each project will likely exclude content from the specific folders where the pages are stored. This is why reuse across projects requires you to use the _includes folder and the common_images folder. (Unfortunately you can’t include an image from the _includes folder.)
Page-level variables
You can also create custom variables in your frontmatter like this:
---
title: Page-level variables
permalink: /page_level_variables/
thing1: Joe
thing2: Dave
---
You can then access the values in those custom variables using the page namespace, like this:
thing1: {{page.thing1}}
thing2: {{page.thing2}}
I haven’t found a use case for page-level variables, but it’s nice to know they’re available.
I use includes all the time. Most of the includes in the _includes directory are pulled into the theme layouts. For those includes that change, I put them inside custom and then inside a specific project folder.