docs: add contributing guidelines

CONTRIBUTING.md

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+# How to contribute to AstroPaper
+
+Thank you for your interest in contributing to **AstroPaper**! We appreciate every contribution, whether you're fixing a typo, improving documentation, or adding a new feature.
+
+## Types of Contributions
+
+There are several ways to contribute to **AstroPaper**, and every contribution counts_ whether it's a PR for a major feature or a small fix.
+
+You can also contribute by leaving review comments on PRs, adding ideas to existing GitHub Issues and Discussions, or helping others by answering questions in GitHub Discussions.
+
+Here’s a summary of the different ways you can contribute:
+
+- [Opening a new issue](#open-a-new-issue)
+- [Submitting PRs](#feature-requests)
+  - [Solving an existing issue](#solving-an-issue)
+  - [Making changes to a blog post](#making-changes-to-a-blog-post)
+- [Helping others by answering issues/discussions](#helping-with-github-issuesdiscussions)
+- [Reviewing existing PRs](#reviewing-existing-prs)
+- [Starting a discussion](#starting-a-discussion)
+
+## Open a new Issue
+
+If you find a bug or problem, first check whether a similar issue already exists. If you don’t find any open issue that addresses the bug/problem you’re facing, feel free to [open a new issue](https://github.com/satnaing/astro-paper/issues/new/choose).
+
+## Feature Requests
+
+If you have an idea for a new feature or enhancement that could improve AstroPaper, we’d love to hear it! Before submitting a new feature request, please:
+
+1. **Check existing discussions/issues**: Review the [Discussions](https://github.com/satnaing/astro-paper/discussions) or [Issues](https://github.com/satnaing/astro-paper/issues) to see if the feature has already been requested or discussed. You can contribute by adding your thoughts or upvoting existing requests.
+2. **Open a new issue**: If you don’t find an existing discussion, you can open a new issue using the [Feature Request Template](https://github.com/satnaing/astro-paper/issues/new?assignees=&labels=enhancement&projects=&template=%E2%9C%A8-feature-request.md&title=%5BFeature+Request%5D%3A+). Be as detailed as possible, describing the problem this feature would solve and how it would benefit AstroPaper users.
+3. **Discuss first**: If you’re unsure whether your idea is feasible or fits the project’s goals, feel free to [start a GitHub Discussion](https://github.com/satnaing/astro-paper/discussions/new/choose) to gather feedback from the community.
+
+## Making PRs (Pull Requests)
+
+### Solving an Issue
+
+Browse through the existing issues to find one that interests you. You can use labels to filter the issues. See the [Label](https://github.com/satnaing/astro-paper/labels) section for more information.
+
+### Making Changes to a Blog Post
+
+For small changes like typos, syntax fixes, or broken links, click the "Suggest Changes" link below the title of any blog post. This will take you to the .md file, where you can make your changes and submit a pull request for review. For more significant changes to a blog post, it’s recommended to open a new issue or discussion first.
+
+## Helping with GitHub Issues/Discussions
+
+GitHub Discussions and Issues are great places to help others. Whether you're a long-time user of AstroPaper or just have experience with a specific problem, we encourage you to answer questions or solve issues when possible.
+
+## Reviewing Existing PRs
+
+You can help by reviewing and providing feedback on open PRs. Different perspectives can be very helpful.
+
+Since AstroPaper doesn’t currently have automated testing, it’s especially useful if you can do manual testing on open PRs and provide feedback.
+
+## Starting a Discussion
+
+If you’re unsure whether your issue warrants a fix or if you just want to share ideas and get feedback, feel free to [start a GitHub discussion](https://github.com/satnaing/astro-paper/discussions/new/choose). It’s a great way to engage with the community.
+