docs: add section for using a different ingress controller (traefik/nginx)

[goseind]

When working with traefik as an ingress controller in one of our clusters (ref https://github.com/EOSC-Future/reana-cnaf-demo), we experienced some issues and thus decided to go with nginx, with this issue we'll add a section about ingress configuration including an example code snipped for nginx.

Further thoughts would be to allow the specification of nginx instead of traefik in the helm chart.