Docs: Snippets

[codeallthethingz]

This PR adds snippet functionality to the docs using the following format:

!snippet[../snippets/repeating-text.md]

repeating-text.md will be inserted before any other processing happens.

Example here: https://thousandbrainsproject.readme.io/v0.0-snippet-test/docs/osscommunication-improvements

[tristanls]

thought (non-blocking): I have a concern that a custom and unique documentation system is being slowly developed instead of adopting a product/commodity documentation system like Markdoc or similar.

[codeallthethingz]

thought (non-blocking): I have a concern that a custom and unique documentation system is being slowly developed instead of adopting a product/commodity documentation system like Markdoc or similar.

Yeah, it's a good thought. Early on I tried rendering just the HTML and sending that to readme, but it stripped it all out.