Add support for checking that all D2 renders are up to date

[nhooyr]

For every .d2 file, we should check that the corresponding .svg exists and is up to date.

[nhooyr]

@alixander is it ok if I change d2-docs render.sh script to then instead copy all rendered .svgs to the generated directory? And not commit that directory.

[nhooyr]

And why do all the svgs in there have .svg2 as the extension instead of .svg?

[nhooyr]

This issue suggests we need something akin to tsconfig.json. Then d2 could just plop all your svgs in your desired folder.

And let you specify which layout you want to layout with. e.g for d2 diagrams in our monorepo, we'd want to test with TALA but in oss, dagre.

[nhooyr]

Whoops, that PR only does formatting now.