docs/feat: a public roadmap for the Ibis project

[lostmygithubaccount]

Please describe the issue

several users have requested a public roadmap for the Ibis project. this helps plan their adoption (e.g. awareness of a large SQLglot refactor or big new features they may be waiting to adopt) and instills confidence in the longevity of the project

we currently have a GitHub project with a roadmap view we should consider making public in a few months after we are confident in our use of it

Code of Conduct

• I agree to follow this project's Code of Conduct

[kszucs]

large SQLglot refactor

For the record, the "large refactor" is about the new relational operations. SQLGlot-ifying the backends is a side effect since the backends must be rewritten and once we are there SQLGlot seems to be more appropriate than alchemy.

[lostmygithubaccount]

for anyone following along, a public GitHub project is up with our rough roadmap: https://github.com/orgs/ibis-project/projects/5/

I'm working on a blog with more prose and explanation, I'll close this issue out with that getting merged

[lostmygithubaccount]

just noting the PR merged the roadmap blog, but didn't publish it yet -- that'll be next week after we announce 8.0