Readme Plugin: Allow the readme card render emojis in Backstage

### Describe the feature

Since the readme card is a tool to provide information to users from the repository readme file, it would be nice that it couldrender emojis in the same way GitHub does.

### Added value

We would improve readability and be consistent with source experience.

### Additional context

Using packages:
@axis-backstage/plugin-readme@^0.9.0
@axis-backstage/plugin-readme-backend@^0.8.0

Start by locating the readme card rendering entry point in @axis-backstage/plugin-readme and review its backend counterpart in @axis-backstage/plugin-readme-backend. Compare the current output with the linked emoji example and identify how the card handles README content; done means emojis render consistently with GitHub in the readme card.

Written by the indexing model from the issue text.