[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/500-feature-request.yml

@@ -0,0 +1,31 @@
+name: 🚀 Feature request
+description: Submit a proposal/request for a new vllm-ascend feature
+title: "[Feature]: "
+labels: ["feature request"]
+
+body:
+- type: markdown
+  attributes:
+    value: >
+      #### Before submitting an issue, please make sure the issue hasn't been already addressed by searching through [the existing and past issues](https://github.com/vllm-project/vllm-ascend/issues?q=is%3Aissue+sort%3Acreated-desc+).
+- type: textarea
+  attributes:
+    label: 🚀 The feature, motivation and pitch
+    description: >
+      A clear and concise description of the feature proposal. Please outline the motivation for the proposal. Is your feature request related to a specific problem? e.g., *"I'm working on X and would like Y to be possible"*. If this is related to another GitHub issue, please link here too.
+  validations:
+    required: true
+- type: textarea
+  attributes:
+    label: Alternatives
+    description: >
+      A description of any alternative solutions or features you've considered, if any.
+- type: textarea
+  attributes:
+    label: Additional context
+    description: >
+      Add any other context or screenshots about the feature request.
+- type: markdown
+  attributes:
+    value: >
+      Thanks for contributing 🎉!