refactor: Refactor modal stories, new modal prop portalElement

[gnapse]

Short description

This is work extracted off of #730, that I deemed useful regardless of whether we adopt Chromatic or not. It is mostly about improvements I had to make to the modal stories to take advantage of Chromatic visual regression tests on modals, but these improvements add value on their own without adopting Chromatic.

Here's a list of the relevant changes:

1. Rewrote modal stories into two modules:
   • MDX for text docs (mostly documenting component APIs and props)
   • .tsx for modal example stories that actually render the modal component
2. Introduce a new portalElement prop on the modal, allowing consumers to customize where in the DOM they want their portal element to be mounted.
3. Added storybooks interaction testing to modal stories.
   • This now makes it so that modals automatically open in modal stories.
   • This opens the door to use storybooks as a place where we run in-browser component tests.
4. Various other small improvements to stories in other components.

PR Checklist

• Added tests for bugs / new features
• Updated docs (storybooks, readme)
• Executed npm run validate and made sure no errors / warnings were shown
• Described changes in CHANGELOG.md
• Bumped version in package.json and package-lock.json (npm --no-git-tag-version version <major|minor|patch>) ref
• Updated all static build artifacts (npm run build-all)

Versioning

A new minor version change, due to the new feature introduced to the modal component.

[rfgamaral]

💭 Nothing against Henning 😅, but doesn't it make more sense to remove this?

[rfgamaral]

Looks good to me 🚀