Communications: add Discord, expand on Discourse (#820)
Communications: add Discord, expand on Discourse (#820)

* Communications: add Discord, expand on Discourse

* move the Diversity statement reference up to the top of the
  page (it's getting a bit hidden as the earlier sections grow)
* add info on joining the Discord server
* expand on the usage of Discourse
* mention and link to the Mailman 3 web gateway

* Fix typo

* Add lock icon to private link

Co-authored-by: Hugo van Kemenade <5a499df1e6a266942706a9de3d4c03795108f8e3@users.noreply.github.com>

Co-authored-by: Hugo van Kemenade <5a499df1e6a266942706a9de3d4c03795108f8e3@users.noreply.github.com>
ezio-melotti/devguide