Docs: Update doc links

[IanVS]

We've created a nebula-specific documentation page at https://docs.defined.net/nebula/, and have improved the config reference docs. This change links to the new site from the README and the issue template.

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[wadey]

I was going to merge this, but it looks like the docs have changed again to https://nebula.defined.net/docs/ etc. Shall we update again before we merge?

[IanVS]

Thanks @wadey, updated.