[Docs] Add issue template (#113)

### What this PR does / why we need it?
Add issue templates.

Most of templates in this PR are from vllm-project/vllm:
https://github.com/vllm-project/vllm/tree/main/.github/ISSUE_TEMPLATE

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?

Test on my local repo by setting default branch to ISSUE_TEMPLATE:
https://github.com/Yikun/vllm-ascend/issues

https://github.com/Yikun/vllm-ascend/issues/new/choose

Closes: https://github.com/vllm-project/vllm-ascend/issues/48

---------

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>

.github/ISSUE_TEMPLATE/750-RFC.yml

@@ -0,0 +1,49 @@
+name: 💬 Request for comments (RFC).
+description: Ask for feedback on major architectural changes or design choices.
+title: "[RFC]: "
+labels: ["RFC"]
+
+body:
+- type: markdown
+  attributes:
+    value: >
+      #### Please take a look at previous [RFCs](https://github.com/vllm-project/vllm-ascend/issues?q=label%3ARFC+sort%3Aupdated-desc) for reference.
+- type: textarea
+  attributes:
+    label: Motivation.
+    description: >
+      The motivation of the RFC.
+  validations:
+    required: true
+- type: textarea
+  attributes:
+    label: Proposed Change.
+    description: >
+      The proposed change of the RFC.
+  validations:
+    required: true
+- type: textarea
+  attributes:
+    label: Feedback Period.
+    description: >
+      The feedback period of the RFC. Usually at least one week.
+  validations:
+    required: false
+- type: textarea
+  attributes:
+    label: CC List.
+    description: >
+      The list of people you want to CC.
+  validations:
+    required: false
+- type: textarea
+  attributes:
+    label: Any Other Things.
+    description: >
+      Any other things you would like to mention.
+  validations:
+    required: false
+- type: markdown
+  attributes:
+    value: >
+      Thanks for contributing 🎉!