misc: enable discussions in the GitHub repository

[Tomoyuki Sakurai]

As a possible contributor, I would like ask questions about the implementation so that I can fix issues I am facing. Discussions is intended for that purpose, but Issues is not.

see https://docs.github.com/en/discussions/quickstart to enable Discussions.

[roleo]

Now is enable.

[Tomoyuki Sakurai]

thanks!