Update release strategy

[nicorikken]

Describe the improvement

Adopt a similar release strategy as for the libraries:

• Release branches (like git flow) for the major versions
• Main branch for continues development, that might be the next release or even further in the future.
• The default branch of the repo should be set to the latest release branch, so people normally don't see the latest branch.
• Adapt the static page deployment to align with this setup.

Additional context

Discussed at meeting 2025-10-07. Current lineair setup is not sufficient. Alternative of separate folders per release in the main branch is an option, but could be higher maintenance and is different than the setup of the libraries.

Expected behavior

• Newcomers arrive at the latest version of the spec, on GitHub and GitHub pages.
• It is easy to push changes to all major release versions.

[nicorikken]

Mike is the recommended plugin for versioning which has specific way of work. We need a variant of the solution. Perhaps there are existing plugins, or perhaps we can still use mike by pointing to the default branch.

[nicorikken]

Related to #88