Update PR template and add checklist workflow

.github/pull_request_template.md

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+## Description
+
+_Add a comprehensive description of proposed changes_
+
+_List associated issue number(s) if exist(s): #6 (for example)_
+
+_Documentation PR (if needed): #1340 (for example)_
+
+---
+
+PR should start as a draft, then move to ready for review state after CI is passed and all applicable checkboxes are closed.
+This approach ensures that reviewers don't spend extra time asking for regular requirements.
+
+You can remove a checkbox as not applicable only if it doesn't relate to this PR in any way.
+For example, PR with docs update doesn't require checkboxes for performance while PR with any change in actual code should have checkboxes and justify how this code change is expected to affect performance (or justification should be self-evident).
+
+Checklist to comply with **before moving PR from draft**:
+
+**PR completeness and readability**
+
+- [ ] I have reviewed my changes thoroughly before submitting this pull request.
+- [ ] I have commented my code, particularly in hard-to-understand areas.
+- [ ] I have updated the documentation to reflect the changes or created a separate PR with update and provided its number in the description, if necessary.
+- [ ] Git commit message contains an appropriate signed-off-by string _(see [CONTRIBUTING.md](https://github.com/intel/scikit-learn-intelex/blob/main/CONTRIBUTING.md#pull-requests) for details)_.
+- [ ] I have added a respective label(s) to PR if I have a permission for that.
+- [ ] I have resolved any merge conflicts that might occur with the base branch.
+
+**Testing**
+
+- [ ] I have run it locally and tested the changes extensively.
+- [ ] All CI jobs are green or I have provided justification why they aren't.
+- [ ] I have extended testing suite if new functionality was introduced in this PR.

.github/workflows/pr-checklist.yml

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+#===============================================================================
+# Copyright 2024 Intel Corporation
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#===============================================================================
+
+name: Check PR Checklist
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, ready_for_review, converted_to_draft]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.ref_name }}-${{ github.event.number || github.sha }}
+  cancel-in-progress: true
+
+jobs:
+  checklist:
+    name: Close all checkboxes before moving from draft
+    timeout-minutes: 5
+    runs-on: ubuntu-24.04
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+    - name: Get pull request details
+      id: pr
+      uses: actions/github-script@v7
+      with:
+        script: |
+          const pr_desc = await github.rest.pulls.get({
+            owner: context.repo.owner,
+            repo: context.repo.repo,
+            pull_number: context.payload.pull_request.number
+          });
+          core.setOutput('body', pr_desc.data.body)
+          core.setOutput('draft', pr_desc.data.draft)
+          core.setOutput('author_type', pr_desc.data.user.type)
+    - name: Check if all checkboxes are checked
+      id: checkboxes
+      env:
+        DESCRIPTION: ${{ steps.pr.outputs.body }}
+      run: |
+        UNCHECKED=$(echo "$DESCRIPTION" | grep -c '\[ \]' || true)
+        echo "unchecked=$UNCHECKED" >> $GITHUB_OUTPUT
+    - name: Fail if not all checkboxes are checked, PR is not draft and author is not a bot
+      if: ${{ (steps.pr.outputs.draft == 'false') && (steps.checkboxes.outputs.unchecked != '0') && (steps.pr.outputs.author_type != 'Bot') }}
+      run: |
+        echo "Unchecked checkboxes: ${{ steps.checkboxes.outputs.unchecked }}"
+        exit 1