Add PR and issue templates from vLLM project

- Add PULL_REQUEST_TEMPLATE.md with vLLM-style PR template
- Add comprehensive issue templates:
  - Bug report template (400-bug-report.yml)
  - Feature request template (500-feature-request.yml)
  - New model support template (600-new-model.yml)
  - Documentation template (100-documentation.yml)
  - Installation template (200-installation.yml)
  - Performance discussion template (700-performance-discussion.yml)
  - Issue template configuration (config.yml)
- Templates adapted for vLLM-omni with multimodal context
- Force added YAML files to override .gitignore exclusion

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,21 @@
+<!-- markdownlint-disable -->
+PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.
+
+## Purpose
+
+## Test Plan
+
+## Test Result
+
+---
+<details>
+<summary> Essential Elements of an Effective PR Description Checklist </summary>
+
+- [ ] The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
+- [ ] The test plan, such as providing test command.
+- [ ] The test results, such as pasting the results comparison before and after, or e2e results
+- [ ] (Optional) The necessary documentation update, such as updating `supported_models.md` and `examples` for a new model.
+- [ ] (Optional) Release notes update. If your change is user facing, please update the release notes draft.
+</details>
+
+**BEFORE SUBMITTING, PLEASE READ <https://github.com/hsliuustc0106/vllm-omni/blob/main/CONTRIBUTING.md>** (anything written below this line will be removed by GitHub Actions)