Add some GitHub templates

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Bug Report
+    url: https://github.com/mono/SkiaSharp/issues
+    about: Create a report in the SkiaSharp repository.
+  - name: Feature Request
+    url: https://github.com/mono/SkiaSharp/issues
+    about: Suggest an idea in the SkiaSharp repository.
+  - name: Questions and/or Discussions
+    url: https://github.com/mono/SkiaSharp/discussions
+    about: Please ask and answer questions in the SkiaSharp repository.

.github/pull_request_template.md

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+**Description of Change**
+
+<!--
+    Describe your changes here.
+-->
+
+**SkiaSharp Issue**
+
+<!--
+    Provide links to SkiaSharp issues here.
+    Ensure that a GitHub issue was created for your feature or bug fix before sending PR.
+-->
+
+Related to https://github.com/mono/SkiaSharp/issues/<issue-number>
+
+**API Changes**
+
+None.
+
+<!--
+List all API changes here (or just put None), example:
+
+Added: 
+- `void skobject_method_name()`
+
+Changed:
+ - `void skobject_old_method_name()` => `void skobject_new_method_name()`
+-->
+
+**Behavioral Changes**
+
+None.
+
+<!--
+    Describe any non-bug related behavioral changes that may change how users app behaves
+    when upgrading to this version of the codebase.
+-->
+
+**Required SkiaSharp PR**
+
+Requires https://github.com/mono/SkiaSharp/pull/<pr-number>
+
+<!--
+    Replace this with the full URL to the skia PR.
+-->
+
+**PR Checklist**
+
+- [ ] Rebased on top of `skiasharp`` at time of PR
+- [ ] Changes adhere to coding standard
+- [ ] Updated documentation

.github/workflows/backport.yml

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+name: Backport
+
+on:
+  pull_request_target:
+    types:
+      - closed
+      - labeled
+
+jobs:
+  backport:
+    name: Backport
+    runs-on: ubuntu-latest
+    # Only react to merged PRs for security reasons.
+    # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
+    if: >
+      github.event.pull_request.merged
+      && (
+        github.event.action == 'closed'
+        || (
+          github.event.action == 'labeled'
+          && contains(github.event.label.name, 'backport')
+        )
+      )
+    steps:
+      - uses: tibdex/backport@v2
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          body_template: >
+             Backport of <%= mergeCommitSha %> from #<%= number %>.
+          head_template: "backport/pr-<%= number %>-to-<%= base %>"
+          label_pattern: "^backport/(?<base>([^ ]+))$"
+          labels_template: "[ \"backport\" ]"
+          title_template: "[<%= base %>] <%= title %>"