The content of the manual in the repo is now out of date. The updated one now lives on jamulus.io. There were various reasons for this, but mainly it gives us the ability to translate it, more control on layout, and there might be some SEO benefit if anyone links to it.

I propose we:

1. Replace the link in Jamulus's help menu to point to the new one (the same goes for Getting Started I think but I'll check).

2. Replace the current content with a message saying it's moved.

If that's OK I'll open a PR.