Implement Alpha Tester Feedback No.1

[x] create optional .css file that can be imported to reset/apply global styles (e.g. rested body margin, apply background color etc.)
[x] OnyxTable: When using OnyxButton inside the header, the button is not clickable
[x] Not all components are exported (mainly support components)
[x] Docs: Mention layout components in getting started guide
[x] Docs: Source code for OnyxNavBar shows <component /> instead of <OnyxNavSeparator />
[x] OnyxNavSeparator: Onyx prefix is missing for source code
[x] OnyxSelect: make more clear that search logic must be implemented on project side
[x] OnyxTable: rename property grid to not mix it up with the "real" layout grid
[x] OnxyTable: does not fit to the grid because it has width: fit-content;
[x] Docs: example for how to override onyx styles (:deep() selector)

[x] OnyxInput/OnyxTextarea: check if <p> should be used instead of <span> for the message
[x] OnyxInput/OnyxTextarea: Label tooltip icon not aligned vertically
[x] OnyxButton: Width changes when set to loading, should keep the same width to prevent layout shifts
[x] OnyxNavBar: add max height for app logo if e..g a custom image with bigger height is provided

[x] OnyxTag: Update description for truncation example that the tag width must be limited for it to work
[x] Storybook truncation decorator: Maybe use style attribute instead so it is shown in the code snippets
[x] OnyxSelect: Support slot to provide fully custom content for an option

SchwarzIT / onyx