[IOAPPX-288] Add Markdown specific typographic scale

[dmnplb]

Short description

This PR adds a new Markdown-specific typographic scale to be used in all Markdown-rendered content in the io-app (detail of a message, service and so on…)

List of changes proposed in this pull request

• Add the new typographic components with the prefix Md to distinguish them from the general typographic components
• Add the new SemiBold weight to ReadexPro to the example app
• Refactor the Typography screen in the example app using the new VStack component

Preview

How to test

1. Launch the example app
2. Go to the Typography screen