Document how to develop with index.html

[joschne]

context
Hot reloading using npm run dev is very slow, since we build stencil with react output target and storybook.

We can simplify this: npx stencil build --dev --watch --serve build-watches stencil and serves packages/design-system-web/src/index.html.

goal
Improve dev experience:

• Create a shortcut for npx stencil build --dev --watch --serve in package.json (maybe even npm run dev?)
• Add a guide in storybook, explaining how to use this command for faster hot reloading with packages/design-system-web/src/index.html
• Remove ...index.html from git and add it to .gitignore
• Add a ...index.html.example to the repo and explain how to use it in a guide in storybook

[joschne]

closing in favor of #62