github: Configure templates for filing issues.

This PR creates templates for filing issues. The templates are
intentionally quite light-weight. Note that I'm specifically not using
forms for creating issues, as the UI for filling out such a form does
not include GitHub's helpful formatting buttons and preview mode.

Follow-up to #25998, pushed as a separate PR so that the original one can pass CI.

This PR creates templates for filing issues. The templates are
intentionally quite light-weight. Note that I'm specifically not using
forms for creating issues, as the UI for filling out such a form does
not include GitHub's helpful formatting buttons and preview mode.

A major goal is to guide users towards starting a CZO conversation
when that's more appropriate than filing a GitHub issue.

Note that the config makes it possible to create a blank issue, which
should be handy for:

* Issues filed by maintainers
* Issues for tracking follow-ups from merged PRs
* Probably some other situations

Because the blank issue option is easy to miss, it should probably be
documented somewhere, but I'm not sure where. We can perhaps start
with a note on CZO.

Relevant CZO threads:

https://chat.zulip.org/#narrow/stream/137-feedback/topic/issues.20link.20in.20description/near/1561110)
https://chat.zulip.org/#narrow/stream/2-general/topic/bug.20report.20management/near/1589141

Also provide external documentation links for situations where
filing an issue is not the best approach.

.github/ISSUE_TEMPLATE/1_discussed_on_czo.md

@@ -0,0 +1,10 @@
+---
+name: Issue discussed in the Zulip development community
+about: Bug report, feature or improvement already discussed on chat.zulip.org.
+---
+
+<!-- Issue description -->
+
+<!-- Link to a message in the chat.zulip.org discussion. Message links will still work even if the topic is renamed or resolved. Link back to this issue from the chat.zulip.org thread. -->
+
+CZO thread

.github/ISSUE_TEMPLATE/2_bug_report.md

@@ -0,0 +1,17 @@
+---
+name: Bug report
+about: A concrete bug report with steps to reproduce the behavior. (See also "Possible bug" below.)
+labels: ["bug"]
+---
+
+<!-- Describe what you were expecting to see, what you saw instead, and steps to take in order to reproduce the buggy behavior. Screenshots can be helpful. -->
+
+<!-- Check the box for the version of Zulip you are using (see https://zulip.com/help/view-zulip-version).-->
+
+**Zulip Server and web app version:**
+
+- [ ] Zulip Cloud (`*.zulipchat.com`)
+- [ ] Zulip Server 7.0+
+- [ ] Zulip Server 6.0+
+- [ ] Zulip Server 5.0 or older
+- [ ] Other or not sure

.github/ISSUE_TEMPLATE/3_feature_request.md

@@ -0,0 +1,6 @@
+---
+name: Feature or improvement request
+about: A specific proposal for a new feature of improvement. (See also "Feature suggestion or feedback" below.)
+---
+
+<!-- Describe the proposal, including how it would help you or your organization. -->

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,14 @@
+blank_issues_enabled: true
+contact_links:
+  - name: Possible bug
+    url: https://zulip.readthedocs.io/en/latest/contributing/reporting-bugs.html
+    about: Report unexpected behavior that may be a bug.
+  - name: Feature suggestion or feedback
+    url: https://zulip.readthedocs.io/en/latest/contributing/suggesting-features.html
+    about: Start a discussion about your idea for improving Zulip.
+  - name: Issue with running or upgrading a Zulip server
+    url: https://zulip.readthedocs.io/en/latest/production/troubleshooting.html
+    about: We provide free, interactive support for the vast majority of questions about running a Zulip server.
+  - name: Other support requests and sales questions
+    url: https://zulip.com/help/contact-support
+    about: Contact us — we're happy to help!