Install fontra pak page: add reference to Windows-specific instructions

[ollimeier]

Just:
I noticed that this page doesn't refer to the Windows-specific instructions: https://docs.fontra.xyz/how-tos/installation/installing-fontra-pak/

TODOs:

• Add link to: https://docs.fontra.xyz/how-tos/installation/installing-fontra-pak-windows/ ?
• to remove the "Have fun"