add MkDocs Plugin to community plugins

[landmaj]

This PR adds MkDocs Plugin to the list of community plugins.

At work we are in the process of switching from PlantUML to D2 and I discovered there was no plugin for MkDocs, which we use to generate documentation for our clients. So I wrote one. You can see a result of using the plugin here.

[gavin-ts]

oh sorry could you sign the commit? #557 (comment)

[landmaj]

Sorry, I didn't notice the requirement before. At least I learned something new today, I've never done it before. 🙂 Should be signed now.

[gavin-ts]

thanks! We've run into this a few times where we don't notice the commit signature missing until after a review approval, so I just made this into its own clear check here: terrastruct/ci#28 and it should make its way into the d2 repo soon.