[chore/github]: Standardize GitHub PR description template

Replace legacy templates with unified pull_request_template.md
(description, type, approach, documentation).

Signed-off-by: Oleksandr Ivantsiv <oivantsiv@nvidia.com>

Author: oleksandrivantsiv

.github/copilot-instructions.md

@@ -76,6 +76,11 @@ Kernel patches are accepted under these conditions:
 - **Upstream first**: Prefer upstream patches — include the original commit hash
 - **Justification**: Explain why a patch is needed and why it can't be done out-of-tree
 - **CI**: Azure pipeline checks must pass
+- **PR description template**: Fill out all sections of the [PR template](.github/pull_request_template.md) when submitting a pull request:
+  - **Description of PR**: Summary of the change, motivation/context, reviewer entry point, and dependencies; reference issues with `fixes #xxxx` / `closes #xxxx`.
+  - **Type of change**: Mark the box(es) that apply — bug fix, new feature, refactor / cleanup, documentation update, test improvement.
+  - **Approach**: Motivation; how you did it; how you verified/tested it; any platform-specific notes.
+  - **Documentation**: Link to wiki / doc updates relevant to new features or test cases.
 
 ## Gotchas
 

.github/pull_request_template.md

@@ -0,0 +1,45 @@
+<!--
+Please make sure you've read and understood our contributing guidelines;
+https://github.com/Azure/SONiC/blob/gh-pages/CONTRIBUTING.md
+
+Please provide following information to help code review process a bit easier:
+-->
+### Description of PR
+<!--
+- Please include a summary of the change and which issue is fixed.
+- Please also include relevant motivation and context. Where should reviewer start? background context?
+- List any dependencies that are required for this change.
+-->
+
+Summary:
+Fixes # (issue)
+
+### Type of change
+
+<!--
+- Fill x for your type of change.
+- e.g.
+- [x] Bug fix
+-->
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Refactor / cleanup
+- [ ] Documentation update
+- [ ] Test improvement
+
+### Approach
+#### What is the motivation for this PR?
+
+#### How did you do it?
+
+#### How did you verify/test it?
+
+#### Any platform specific information?
+
+### Documentation
+<!--
+(If it's a new feature, new test case)
+Did you update documentation/Wiki relevant to your implementation?
+Link to the wiki page?
+-->