Add some issue templates (#2225)

ref: https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository

example of how this is supposed to look: https://github.com/flutter/flutter/issues/new/choose

.github/ISSUE_TEMPLATE/01_explorer_bug.yaml

@@ -0,0 +1,44 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+name: Carbon Explorer bug
+description: >
+  Report a bug with the Carbon Explorer interpreter. This is what's provided at
+  [Compiler Explorer](https://carbon.compiler-explorer.com).
+labels: [explorer]
+body:
+  - type: markdown
+    attributes:
+      value: >
+        **Attention:** If this is a _question_, please use either [GitHub
+        Discussions](https://github.com/carbon-language/carbon-lang/discussions)
+        or [Discord](https://discord.gg/ZjVdShJDAs).
+  - type: textarea
+    id: desc
+    attributes:
+      label: >
+        Description of the bug:
+  - type: textarea
+    id: repro
+    attributes:
+      label: >
+        What did you do, or what's a simple way to reproduce the bug?
+      description: >
+        Please provide example code and errors; a shortlink to the execution on
+        [Compiler Explorer](https://carbon.compiler-explorer.com) can also help.
+  - type: textarea
+    id: expected
+    attributes:
+      label: >
+        What did you expect to happen?
+  - type: textarea
+    id: actual
+    attributes:
+      label: >
+        What actually happened?
+  - type: textarea
+    id: extras
+    attributes:
+      label: >
+        Any other information, logs, or outputs that you want to share?

.github/ISSUE_TEMPLATE/02_toolchain_bug.yaml

@@ -0,0 +1,43 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+name: Toolchain bug
+description: >
+  Report a bug with the toolchain.
+labels: [toolchain]
+body:
+  - type: markdown
+    attributes:
+      value: >
+        **Attention:** If this is a _question_, please use either [GitHub
+        Discussions](https://github.com/carbon-language/carbon-lang/discussions)
+        or [Discord](https://discord.gg/ZjVdShJDAs).
+  - type: textarea
+    id: desc
+    attributes:
+      label: >
+        Description of the bug:
+  - type: textarea
+    id: repro
+    attributes:
+      label: >
+        What did you do, or what's a simple way to reproduce the bug?
+      description: >
+        Please provide example code and errors; a shortlink to the execution on
+        carbon.compiler-explorer.com can also help.
+  - type: textarea
+    id: expected
+    attributes:
+      label: >
+        What did you expect to happen?
+  - type: textarea
+    id: actual
+    attributes:
+      label: >
+        What actually happened?
+  - type: textarea
+    id: extras
+    attributes:
+      label: >
+        Any other information, logs, or outputs that you want to share?

.github/ISSUE_TEMPLATE/03_issue_for_leads.yaml

@@ -0,0 +1,37 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+name: Issue for leads
+description: >
+  Ask the leads for feedback on an important issue, such as a language design
+  choice.
+labels: [issue for leads]
+body:
+  - type: markdown
+    attributes:
+      value: >
+        **Attention:** If this is a _question_, please use either [GitHub
+        Discussions](https://github.com/carbon-language/carbon-lang/discussions)
+        or [Discord](https://discord.gg/ZjVdShJDAs).
+  - type: textarea
+    id: summary
+    attributes:
+      label: >
+        Summary of issue:
+      description: >
+        Provide a short, one-paragraph summary of the issue.
+  - type: textarea
+    id: detail
+    attributes:
+      label: >
+        Details:
+      description: >
+        Provide detailed information about the issue. For example, on a language
+        design choice, it can help to provide various options with a summary of
+        trade-offs.
+  - type: textarea
+    id: extras
+    attributes:
+      label: >
+        Any other information that you want to share?

.github/ISSUE_TEMPLATE/config.yaml

@@ -0,0 +1,12 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+blank_issues_enabled: true
+contact_links:
+  - name: I have a question about Carbon
+    url: https://github.com/carbon-language/carbon-lang/discussions/categories/getting-started
+    about: Ask questions about Carbon in GitHub Discussions.
+  - name: I want to join the Discord
+    url: https://discord.gg/ZjVdShJDAs
+    about: Discord is where a lot of design discussion occurs.