docs: add note about non-schema front matter

Adds a note to the Markdown docs about non-schema front matter.

Also adds a link to the settings file in GitHub to show the default
values

Author: johnfraney

docs/content/configuration/settings.md

@@ -20,3 +20,5 @@ The setting hierarchy is:
 See the `Settings` type for Blurry's available settings:
 
 @python<blurry.settings.Settings>
+
+The default values are visible at <https://github.com/blurry-dev/blurry/blob/main/blurry/settings.py>

docs/content/content/markdown.md

@@ -20,13 +20,18 @@ name = "About Blurry"
 abstract = "Learn about Blurry, a static site generator build for page speed and SEO"
 datePublished = 2023-01-07
 image = "../images/blurry-logo.png"
+_is_this_available_in_a_template = True
 +++
 
 # About Blurry
 
 Regular Markdown content can go here.
 ```
 
+:::{info}
+Variables that are not in a Schema.org type but are useful in templates should start with the `FRONTMATTER_NON_SCHEMA_VARIABLE_PREFIX` [setting](../configuration/settings.md), which defaults to an underscore (`_`).
+:::
+
 ## Customizations
 
 On top of [Mistune's built-in plugins](https://mistune.lepture.com/en/latest/plugins.html), Blurry ships with a number of Markdown customizations.