How can a user point his site to my SaaS website?

You can check out their documentation:
https://caddyserver.com/docs/caddyfile

For my own set-up, I have an EC2 instance with caddy running as a service (https://caddyserver.com/docs/install#linux-service), then I have a CaddyFile which looks a little something like this - where fizz.com and buzz.com are client domains, and my_origin_server.com is my app server. I then have tls on _demand, which means that a lets encrypt cert is requested the first time a request comes in from one of these domains. The benefit of doing it on demand means we can have this all set-up in advance before the client has updated their DNS records.

fizz.com, www.fizz.com,
buzz.com, www.buzz.com
{
reverse_proxy my_origin_server.com:1234
tls {
on_demand
}
}

I'd want them to point their custom domain as a CNAME record to myappdns.com and this would be pointed to the IP Address of my server

CName will work fine for subdomains (ie. foo.clientsite.com -> myappdns.com), however apex/naked domains will have to point at the IP via an A record, or you can use a new fangled AName record which isn't massively support by DNS providers. (ie. clientsite.com -> 23.23.23.23)

Another thing to think about is setting up SSL for your clients (which is why I mentioned the reverse proxy).

Also, if I provision my own http server, I should be doing tons of maintenance and devops for it. (uptime, crash, etc)

If you're referring to the reverse proxy, I believe there is a maintained docker image available for caddy.

I'm afraid that I'm not aware of any backend platform that takes care of all of this for you.

Hey, you need to setup NGINX on Digitalocean and make it accept requests from yourdomain.com via the server_name setting. You can set a wildcard for server_name so that it will accept any domain