docs: Replace community slack links with discord

[matevz]

No description provided.

[kostko]

Note that this will need to be backported (or source branch changed in docs) as we are currently showing the stable branch in docs.

[matevz]

Note that this will need to be backported (or source branch changed in docs) as we are currently showing the stable branch in docs.

Actually the two changed files are not integrated into docs.oasis.dev, so this won't be needed. The remainder of the slack links in docs is fixed here oasisprotocol/docs#103.