Storybook: Migrate stories contents regarding GitHub flavored markdown

[mfranzke]

As a follow up to #2093 we'd have to migrate some documentations contents:

In MDX1 you had the option of using GitHub flavored markdown.
Storybook 7.0 uses MDX2 for compiling MDX, and thus no longer supports GFM out of the box.
Because of this you need to explicitly add the GFM plugin in the addon-docs options:
https://storybook.js.org/docs/react/writing-docs/mdx#lack-of-github-flavored-markdown-gfm
We recommend you follow the guide on the link above, however we can add a temporary storybook addon that helps make this migration easier.
We'll install the addon and add it to your storybook config.

[dirkesquire]

I wish the documentation contents would tell us if we really needed to add GFM support. What happens if we don't add it? Is adding it recommended? The help seems to be telling us to add it, when it sounds like it is not really needed. (but I could be wrong).