Add changelog for Godot 4.2

We're changing the format of the changelog as the previous one based on
Keep a Changelog required too much manual work to maintain.

We now link to relevant resources, notably our online interactive
changelog, and copy the grouped changelog generated by this tool.

It's a bit less curated than before, as we used to remove entries which
aren't that relevant to end users, and sort them under Added/Changed/
Removed/Fixed, but this was done manually and thus a huge chore.

Past changelogs are also removed and replaced with links to the
changelogs in their respective feature branches.

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