docs: move changelog from pure markdown to sphinx based docs

[consideRatio]

I wanted to get this done as I want to easily be able to link to the configuration reference various sections, but if the changelog is a pure markdown file, separate from the z2jh docs, we must use full hardcoded links etc that makes it messier.

Closes #2221.

To be resolved

• Auto-generated header links ignore mis-anchor headers that start with a number executablebooks/MyST-Parser#527

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EDIT: linkcheck failure unrelated and fixed in #2775.

[consideRatio]

I'll go for a self-merge on this as it is mostly a technical change only related to documentation and I think the changelog for 2.0.0 can be finalized now - but should be adjusted based on this PR to better link out to the various schema references.