use global contributing guide

Remove the per-project files so we don't have to
keep them in sync. GitHub's UI links to everything
except the contributing guide, so add a section
about that to the readme.

.github/ISSUE_TEMPLATE/config.yml

@@ -3,9 +3,9 @@ contact_links:
   - name: Security issue
     url: https://github.com/pallets/flask/security/advisories/new
     about: Do not report security issues publicly. Create a private advisory.
-  - name: Questions
+  - name: Questions on GitHub Discussions
     url: https://github.com/pallets/flask/discussions/
     about: Ask questions about your own code on the Discussions tab.
-  - name: Questions on
+  - name: Questions on Discord
     url: https://discord.gg/pallets
     about: Ask questions about your own code on our Discord chat.