Switch to supercollider.online as domain

[capital-G]

Also see

• https://scsynth.org/t/moving-the-supercollider-webpage/
• Obtain a new official URL which is not owned by GitHub supercollider#6114

Although GitHub is a convenient platform for developers, SuperCollider should not be too tied to GitHub.
In particular, any external communication should always be independent of GitHub or any other coding platform, as history has shown for example via https://supercollider.sourceforge.net/. Using a self-owned and platform independent domain creates a more resilient address for the SuperCollider community.
Setting up a domain is an easy task and does not consume too many resources, so I decided to donate an official domain to SuperCollider called https://supercollider.online (I also bought the address https://supercollider.dev as a "backup").

I am happy to share any admin rights of the URL with the maintainers through OVH IAM management (see scsynth post), in order to be more resilient if, for example, I will be absent from the community indefinitely.

There are already some other websites which are operating under this domain, such as

• https://docs.supercollider.online/ - see https://github.com/capital-G/sc-docs
• https://baryon.supercollider.online/ - see https://github.com/capital-G/baryon
• https://quarks.supercollider.online/ - see https://github.com/capital-G/quarks-web -will soon be deprecated soon in favor of baryon

I already setup the A record for the domain, so that once the branch is deployed the website would be available under https://supercollider.online.

If deployed, the "old" domain https://supercollider.github.io/ will automatically redirected to the new domain https://supercollider.online, see https://capital-g.github.io/quarks-web/ which now redirects automatically to https://quarks.supercollider.online/

[dyfer]

This PR is mostly to agree whether we'll use supercollider.online as the new "official" domain, particularly for the website.