Change the discussion and home pages following the discontinuation of discuss issues

[JonathanGregory]

Following our discussion and decision to discontinue the use of issues for comments, questions and announcements, I've created a PR to make website updates to suit this new arrangement.

• On the home page, I've added a link near the top for "Adding a question about how to use CF", to open a new GitHub Discussion in the Q&A category. The other changes are all on the discussion page:

• There is a new first section of this page about questions, comments and announcements in GitHub Discussion, with links to the open discussions in each category, and links to start new ones.

• There's a sentence about watching the discuss repo if you want to receive notifications from GitHub Discussions. Unfortunately I don't know a way to give a link for the "watch" choices, because it's a drop-down menu.

• The second section is concerned with issues, as before, but now the standard names etc. issues are in the new vocabularies repo. I have added links to open a new issue for each repository, and to get lists of closed issues, optionally categorised by reason for closure in the conventions repo. There's a sentence about watching any of these repos if you want notifications of issues.

• In the final section, about archive links, I've added a note that the currently open issues in discuss will be converted to Discussions. We can remove this note once that's been completed.

Here you can see what the home page would look like, and here the discussion page, once these changes have been made. Thanks to Antonio for making GitHub Pages do the rendering of a fork of the website repo.

Please could someone approve these changes and merge the PR if they are OK? I have classed this as an update (which can be done immediately) rather than an enhancement, but if anyone thinks this change needs further consideration before it's implemented, please say so. Updates to the website must be approved by a member of the Conventions Committee and Standard Names Committee, the Governance Panel, or someone to whom they have delegated this responsibility. Updates should reflect decisions that have already been made by the Governance Panel or one of the Committees. They can be merged immediately after approval.

Thanks, Jonathan