Introduce variables for versions

[inteon]

Introduces a new variable format [[VAR::cert_manager_latest_version]] which can be used inside codeblocks and reads variables from the variables.json file. The solution is based on https://github.com/facebook/docusaurus/issues/395#issuecomment-1727777067 and works around an issue with MDX which does not allow you to have variables in codeblocks (https://github.com/orgs/mdx-js/discussions/2288).

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[maelvls]

Hey, great finding!

Can you update the Release Process document to reflect that? I think we can remove a lot of sed that I had added a while back?

/hold if you want to update the release process. /lgtm /approve

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[inteon]

I'll create a seperate PR that updates the release doc. @maelvls /unhold