Update development docs for new gatsby site

[emplums]

I noticed that the docs are out of date now that we've moved to doctocat! I've updated a few things.

@shawnbot I have a few questions!

• Is storybook still the defacto way to work on new components or update an existing component? It seems like it's no longer functioning 😬. The main issue was that the folder we pointed to to grab markdown to turn into stories was incorrect, so I tried changing that, but now I'm getting this error:

WARNING in ./docs/content/utilities/index.md 1:2
Module parse failed: Assigning to rvalue (1:2)
You may need an appropriate loader to handle this file type.
> ---
| title: Utilities
| path: utilities/index

Not sure if it's the front matter specifically that's causing an issue or if we need to add a loader for md in general?

• Is the test-urls script still relevant? If so I think we need to update that as well

I can make issues for these things if so!

/cc @primer/ds-core

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[shawnbot]

@emplums thanks for this! I'd like to merge this soon and answer your questions:

• Is storybook still the defacto way to work on new components or update an existing component? It seems like it's no longer functioning 😬.

Eek, yeah — let's file an issue for that. I think these days we're just using the docs site to test our components, so maybe Storybook isn't worth maintaining anymore?

• Is the test-urls script still relevant? If so I think we need to update that as well

It's really only valid if we care about preserving redirects from the styleguide, which I think we still do. It does appear to still work, though, so I think we're good for now. When we do get around to shutting down the styleguide, we should probably refactor this test to ensure that we're not accidentally breaking any existing URLs, and we can do that by testing the paths that Gatsby exports.