Start using Discussions and remove gitter

[avgustinmm]

Hi all,
as proposed by fkromer in #2738, maybe it's good to start using project's Discussions.
For me it will bring some benefits:

• move questions from Issues to Discussions thus having clearer view on what real issues are
• switch from gitter to Discussion thus concentrate all communication into github
• more actively involve the community into any planned changes & updates

What do you think about?

• starting to exploit Discussions
• removing gitter as duplicated communication channel?