Improve clarity on how to propose a new hook

[dennispatterson]

The question came up 3 times at the May 2020 connectathon as to how to propose a new hook. A couple of those questions pertained to where should they go to create a hook proposal, and one was more processed-focused around whether to go on Zulip or create a PR.

• The specification has a hook template - https://cds-hooks.org/hooks/template/
• There's a separate wiki for previously discussed hook proposals, so there's a bit of ambiguity whether that should be used - https://github.com/cds-hooks/docs/wiki/Proposed-Hooks
• There's another incomplete wiki for hook design best practices - https://github.com/cds-hooks/docs/wiki/Hook-Design-Best-Practices

My guidance in discussions was

• If you're just getting a feel for whether something is appropriate as a hook, you could solicit input on Zulip
• If you have a proposal, you can open a PR using the hook template, also remembering to add it to the sidebar in the mkdocs YAML, then additionally cross-posting to Zulip, since that's part of the Hook Maturity Model
  • Example: Propose new appointment-book hook #464

Based on this, we could clean up some of those remnant wiki pages and either create a dedicated Wiki page to creating a new hook or a Contributing page on cds-hooks.org with clearer instructions