fix(README): dollar sign and missing >

[mcanouil]

This PR:

• removes the dollar sign which is copied when using the "copy" button on GitHub.
• fix missing > in shorcodes
• rebase: after #1 fix

[mcanouil]

To note, the dollar is also in the install command of:

• https://github.com/quarto-ext/grouped-tabsets
• https://github.com/quarto-ext/fancy-text

[mcanouil]

Closes this in favour of #10