Add "deprecated functions" section to builtin reference docs

[anderseknert]

While we advice against using these functions, it still may happen that users are trying to understand an older policy found in the wild, and not having any (pun not intended!) place to read about all (I swear, not intended!) the deprecated builtins leaves them out in the dark. We should add a "deprecated functions" section to the builtin reference, and for each builtin provide:

• The name of the builtin
• What to use instead

The "what to use instead" information should also be exported to the builtin_metadata.json file, so that we can use it to present that information in e.g. the VS Code or IntelliJ IDEA plugin.

[stale]

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