Replace remark-includes with MDX imports

[ptgott]

The Docusaurus site currently configures the remark-includes plugin to transform content before we build the site. This uses a lot of custom logic that we can replace with MDX-native imports, including both importing Markdown and importing code snippet content.

We can write a script that replaces our (!path!) syntax with:

• An import statement at the top of the including page
• A React-style component tag pair at the site of the (!path!) expression

Then we can run the script once for all of our partials.

We can also replace our home-grown parameter substitution logic with something like {props.name} in the Docusaurus Markdown import example.

Ideally, this will also mean that we don't have to invoke remark-includes in order to lint partials, since remark won't exit with an error as it does when it encounters our {{ param="value" }} syntax, and can lint these files as regular MDX documents. This will reduce the time it takes our remark linter to run.