Link to self-hosting guide is broken on https://www.monicahq.com/

[Jan Ignatius]

Steps to reproduce:

1. Browse to monicahq.com
2. Try clicking the link embedded in the call-to-action text "No credit card required. Or install it on your own server."
3. End up with a 404 link on Github

Expected behavior:
Link should point to https://github.com/monicahq/monica/blob/4.x/docs/installation/readme.md instead.