[Docs] Add "Open Projects" page

There are a number of cool projects/ideas (Arc, ImportVerilog, SystemC, LEC, Model Checking, Synthesis, HLS, Rust bindings and so on) in CIRCT but I think currently it's not much visible from outsides. For open projects it would be nice to create a page like https://mlir.llvm.org/getting_started/openprojects/.

Use the MLIR Open Projects page linked in the issue as the reference, then inspect CIRCT's documentation structure and navigation to find where a corresponding page belongs. Gather the listed open projects and make the new page discoverable from the documentation; done means CIRCT visibly presents its open projects and ideas.

Written by the indexing model from the issue text.