ci: automate gsoc label assignment

- Added a PR template by referring existing templates in preCICE repos.
- Added workflow to automatically assign GSoC labels via PR template checkbox.

Author: aadhil2k4

.github/pull_request_template.md

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+## Main Changes of this PR
+
+
+## Motivation and additional information
+
+<!--
+Short rational why preCICE needs this change. If this is already described in an issue a link to that issue (closes #123) is sufficient.
+-->
+
+## Steps to Test
+
+
+## Screenshots (if applicable)
+
+
+## Author's checklist
+
+* [ ] I ran the website locally and verified the changes.
+* [ ] I ensured links and formatting render correctly.
+* [ ] I added screenshots if UI/visual changes are introduced.
+* [ ] I linked the related issue (if applicable).
+* [ ] I squashed commits that belong together.
+* [ ] This PR is a part of Google Summer of Code (GSoC)
+
+## Reviewers' checklist
+
+<!-- Tag people next to each point and add points for specific questions -->
+
+* [ ] Does the changelog entry make sense? Is it formatted correctly?
+* [ ] Do you understand the code changes?
+
+<!-- add more questions/tasks if necessary -->

.github/workflows/auto-label-gsoc.yml

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+name: Automatic labelling of GSoC related PRs
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize]
+
+jobs:
+  label-gsoc:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Add GSoC label if checkbox is checked
+        uses: actions/github-script@v7
+        with:
+          script: | 
+            const body = context.payload.pull_request.body || "";
+            const gsocChecked = body.includes("[x] This PR is a part of Google Summer of Code (GSoC)") ||
+                body.includes("[X] This PR is a part of Google Summer of Code (GSoC)");
+            if(gsocChecked) {
+              await github.rest.issues.addLabels({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  issue_number: context.payload.pull_request.number,
+                  labels: ["GSoC"]
+              });
+            }