docs: add issue templates, contributing file

Signed-off-by: Slendi <slendi@socopon.com>

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,41 @@
+name: Feature request
+description: Propose an enhancement to smath
+title: "feat: "
+labels:
+  - enhancement
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for suggesting an improvement.
+        Clear use cases help us evaluate feature requests quickly.
+
+  - type: textarea
+    id: problem
+    attributes:
+      label: Problem statement
+      description: What problem are you trying to solve?
+      placeholder: Describe the limitation or pain point.
+    validations:
+      required: true
+
+  - type: textarea
+    id: proposal
+    attributes:
+      label: Proposed solution
+      description: Describe the API or behavior you want.
+      render: cpp
+    validations:
+      required: true
+
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered
+      description: What other approaches did you consider?
+
+  - type: textarea
+    id: impact
+    attributes:
+      label: Impact and compatibility
+      description: Any concerns around performance, API compatibility, or migration?