Rename `<Markdown>` to `<RichSlot>`

[atinux]

Is your feature request related to a problem? Please describe

Markdown is too confusing when creating a components library to be used in MDC but also outside in other Vue files.

Describe the solution you'd like

• Renaming <Markdown> to <RichSlot>
• Creating <Markdown> as an alias of <RichSlot> + warning (or see with TS warning) about deprecation

[pi0]

By renaming, please also consider adding Content prefix to component names as <ContentRichSlot>, following module best practices.