docs: Added initial PR template with directions for doc only changes and squash merges [no ci] (ggml-org#7700)

This commit adds pull_request_template.md and CONTRIBUTING.md . It focuses on explaining to contributors the need to rate PR complexity level, when to add [no ci] and how to format PR title and descriptions.

Co-authored-by: Brian <[email protected]>
Co-authored-by: compilade <[email protected]>

Author: 3 people

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

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+- Self Reported Review Complexity:
+    - [ ] Review Complexity : Low
+    - [ ] Review Complexity : Medium
+    - [ ] Review Complexity : High
+- [ ] I have read the [contributing guidelines](CONTRIBUTING.md)

CONTRIBUTING.md

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+# Contributing Guidelines
+
+## Checklist
+
+* Make sure your PR follows the [coding guidelines](https://github.com/ggerganov/llama.cpp/blob/master/README.md#coding-guidelines)
+* Test your changes using the commands in the [`tests`](tests) folder. For instance, running the `./tests/test-backend-ops` command tests different backend implementations of the GGML library
+* Execute [the full CI locally on your machine](ci/README.md) before publishing
+
+## PR formatting
+
+* Please rate the complexity of your PR (i.e. `Review Complexity : Low`, `Review Complexity : Medium`, `Review Complexity : High`). This makes it easier for maintainers to triage the PRs.
+    - The PR template has a series of review complexity checkboxes `[ ]` that you can mark as `[X]` for your conveience. Refer to [About task lists](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/about-task-lists) for more information.
+* If the pull request only contains documentation changes (e.g., updating READMEs, adding new wiki pages), please add `[no ci]` to the commit title. This will skip unnecessary CI checks and help reduce build times.
+* When squashing multiple commits on merge, use the following format for your commit title: `<module> : <commit title> (#<issue_number>)`. For example: `utils : Fix typo in utils.py (#1234)`