Roadmap in docs is outdated

[pavithraes]

The roadmap docs page describes work planned in 2023 for v0.9 and v1.0 releases. Since these are now complete, the document is out of date.

We can consider removing the page and reinstating it once a new roadmap is finalized.

The GitHub project boards for the workstreams might still be useful and relevant, so we can also update the page to only have an overview of the workstreams.