Add supported HTML elements in the Styleguide

[Jan-Eimertenbrink]

We need a dedicated place in the Styleguide that explains which HTML elements are supported inside remote components. This helps developers understand what they can safely use.

Supported elements at the moment are

• svg
• circle
• g
• path
• strong
• s
• ul
• ol
• li
• small
• br

[kati-ux]

Depending on what exactly is being documented, I would see this topic fitting into the Get Started section. The Get Started area is planned to be expanded in the near future to include things like: how the components are structured and what you need to know to actually work with them.