clearer instructions on adding domains and custom domains

[edublancas]

customers are getting confused on how to add the DNS records to register a custom domain. I think the best solution is to provide generic instructions for DNS providers and add specific instructions for popular providers, as they often have some quirks.

for example, when registering a subdomain, namecheap automatically adds the base domain to the CNAME name (see this). we already have a note on the docs but seems like it's unclear

the same happens with GoDaddy, if someone wants to register subdomain.example.com, the CNAME name should be just subdomain, and for the SSL CNAME record, they also need to remove the base domain (example.com), this is how a correct configuration looks like:

we need to write separate sections for godaddy and namecheap since these two are popular

[edublancas]

fixed by #289