Add a simple maintenance page

[ryuukk]

This could be useful when a service such as the forums is down

A simple redirect on the DNS/domain that points to https://dlang.org/maintenance.html and it should be good!

[CyberShadow]

This could work if the DNS host supports redirecting to a web URL (so pointing the record at a landing page and then doing a HTTP redirect). Not sure if ours has that feature?

[CyberShadow]

@mdparker What do you think, does this make sense?

[mdparker]

I don't see a problem with it in principle, but I don't think it's practical until we get all of the subdomains under one roof.