[MAINT] Add the 2024/2025 roadmap

[peytondmurray]

Description

This PR adds a ROADMAP.md. It's currently a preliminary 2024 roadmap, but we can reuse this in later years as well.

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

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[trallard]

Left a number of comments, questions, and request for changes.
Can you please move this to the community section of the docs, please?

[peytondmurray]

Arrgh, yes, it looks like the nesting got messed up - will change this now.

[trallard]

Small changes requested, feel free to commit and merge @peytondmurray