chore: automate plugin page headings

[SethFalco]

Makes two major changes to our documentation.

Now we check the front matter, and determine if we're currently rendering a plugin page and inserts the PluginUsage, PluginParams, and PluginDemo components.

We specified these components in the docs pages themselves before, because Docusaurus won't generate TOC entries from headings in React components. However, we hack around this by inserting them in the TOC component itself if it's a plugin page.

It's a little hacky, but makes the documentation much cleaner and reduces human-error, which has already caused issues in the documentation before.

The other change is that this drops the implementation heading for each plugin, and instead inserts an entry before to the "Edit this page on GitHub" link to "Read the implementation". This is much cleaner.