Disable indexing on changelog

[ethanpalm]

Documentation changes

This PR adds noindex: true to the changelog.

Because the changelog represents moments in time and not an always up to date source of truth, we should remove it from the assistant's indexing. This should help the assistant stop referencing deprecated items that are in the changelog, but not the rest of the docs like mint.json and the AI widget.

However, this also disables SEO indexing, so cc @tiffany-mintlify to make sure there aren't unintended consequences of that.

Ultimately, there shouldn't be an expectation that the changelog is evergreen. Things will change over time and we shouldn't be trying to maintain accuracy in the changelog. Both for transparency and because that is just duplicating the docs, but in a worse way.

Part of https://linear.app/mintlify/issue/DOC-122/formalize-changelog-process

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