Add Storybook DocsPage for Vue

[shilman]

I added Storybook DocsPage which will ship with first-class Vue support in 5.3.

If we can improve the docs a bit, I'd like to use this as an example in the Storybook 5.3 release.

A few things I'm hoping to add:

• some intro MDX docs
• Component description comments (blocked on vue-docgen-api cc @Aaron-Pool)
• Support for subcomponents, e.g. AlertIcon, AlertTitle, AlertDescription) (coming in Addon-docs: Multi-component props tables on DocsPage storybookjs/storybook#7811)
• Potentially support for rich props tables (coming in Addon-docs: Rich props table UI storybookjs/storybook#8828 cc @patricklafrance)

Thanks to @Aaron-Pool @pocka @elevatebart for help out!

[shilman]

@Aaron-Pool FYI I tried to add an MDX intro story and ran into the following error, which reminds me of this problem we ran into in the MDX storyshots PR: storybookjs/storybook#8557 (comment)

Error: This API has been removed. If you're looking for this functionality in Babel 7, you should import the '@babel/helper-module-imports' module and use the functions exposed from that module, such as 'addNamed' or 'addDefault'.
at File.addImport
...

I'll open a separate PR for that after this one gets merged and it would be great if we could get to the bottom of it. MDX is working in Storybook's vue-kitchen-sink example somehow.

[shilman]

@Aaron-Pool I added in subcomponents. ButtonGroup/Button works:

Unfortunately, AlertIcon, AlertTitle, AlertDescription don't seem to have docgen info. Is this something you might be able to look into?

[Aaron-Pool]

@shilman yup, I'll try to get to that today!