Digging into www, didn't found an answer: I want to know how I can use cert-manager with haproxy-ingress and lets encrypt.
Any documentation / guidelines?
you can try installing cert manager provided by jetstack, can be found here and then you need to follow the steps mentioned in this stackoverflow post and this will get things sorted for you.
An internal acme-challenge will be done by cert manager and once you patch the secret name, mentioned in the certificate to the TLS of ingress then certificate status will get ready state, Note that the secret will get created automatically, you need not create it