Add links to CF Discussions and update description of use of issues

[JonathanGregory]

I have prepared a pull request to change the CF website in order to direct visitors to CF Discussions for announcements, comments, ideas and questions. The two affected webpages can be previewed as they would appear when live: CF home page, discussions page. In addition, on the discussion page, I have added links to produce lists of closed conventions issues in three categories (change agreed, agreement not to change, dormant).

I believe these changes can be treated as a update to be made with immediate effect, rather than as an enhancement to be agreed, because they follow a decision of the governance panel.

Please could a member of the governance panel or the information management team check the PR, and merge if there are no corrections or additions to be made? Thanks.