Tooling: Adds PR Template

.github/PULL_REQUEST_TEMPLATE.md

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+<!--  Thanks for sending a pull request! Here are some tips for you:
+
+1. If this is your first time contributing to Gateway API Inference Extension, please read our
+   developer guide (https://github.com/kubernetes-sigs/gateway-api-inference-extension/blob/main/docs/dev.md)
+   and our community page (https://gateway-api-inference-extension.sigs.k8s.io/contributing/).
+2. If this is your first time contributing to a Kubernetes project, please read
+   our contributor guidelines:
+   https://git.k8s.io/community/contributors/guide/first-contribution.md#your-first-contribution
+3. Please label this pull request according to what type of issue you are
+   addressing, especially if this is a release targeted pull request. For
+   reference on required PR/issue labels, read here:
+   https://git.k8s.io/community/contributors/devel/sig-release/release.md#issuepr-kind-label
+4. If you want *faster* PR reviews, read how:
+   https://git.k8s.io/community/contributors/guide/pull-requests.md#best-practices-for-faster-reviews
+5. If the PR is unfinished, see how to mark it:
+   https://git.k8s.io/community/contributors/guide/pull-requests.md#marking-unfinished-pull-requests
+6. If this PR is a new design proposal, please review existing design docs for guidance:
+   https://github.com/kubernetes-sigs/gateway-api-inference-extension/tree/main/docs/proposals
+-->
+
+**What type of PR is this?**
+<!--
+Add one of the following kinds:
+/kind bug
+/kind cleanup
+/kind documentation
+/kind feature
+/kind gep
+/kind test
+
+Optionally add one or more of the following kinds if applicable:
+/kind deprecation
+/kind failing-test
+/kind flake
+/kind regression
+/area conformance-test
+/area conformance-machinery
+-->
+
+**What this PR does / why we need it**:
+
+**Which issue(s) this PR fixes**:
+<!--
+*Automatically closes linked issue when PR is merged.
+Usage: `Fixes #<issue number>`, or `Fixes (paste link of issue)`.
+-->
+Fixes #
+
+**Does this PR introduce a user-facing change?**:
+<!--
+If no, just write "NONE" in the release-note block below.
+If yes, please enter a release note below:
+-->
+```release-note
+
+```