chore: add contributing guide

apps/documentation/pages/contributing/_meta.json

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+{
+  "index": {
+    "title": "Get started",
+    "theme": {
+      "sidebar": false
+    }
+  }
+}

apps/documentation/pages/contributing/index.mdx

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+---
+title: Contributing Guide
+description: Set up Documenso using Docker for local development.
+---
+
+# Contributing to Documenso
+
+If you plan to contribute to Documenso, please take a moment to feel awesome ✨ People like you are what open source is about ♥. Any contributions, no matter how big or small, are highly appreciated.
+
+## Before getting started
+
+- Before jumping into a PR be sure to search [existing PRs](https://github.com/documenso/documenso/pulls) or [issues](https://github.com/documenso/documenso/issues) for an open or closed item that relates to your submission.
+- Select an issue from [here](https://github.com/documenso/documenso/issues) or create a new one
+- Consider the results from the discussion on the issue
+- Accept the [Contributor License Agreement](https://documen.so/cla) to ensure we can accept your contributions.
+
+## Taking issues
+
+Before taking an issue, ensure that:
+
+- The issue has been assigned the public label
+- The issue is clearly defined and understood
+- No one has been assigned to the issue
+- No one has expressed intention to work on it
+
+You can then:
+
+1. Comment on the issue with your intention to work on it
+2. Begin work on the issue
+
+Always feel free to ask questions or seek clarification on the issue.
+
+## Developing
+
+The development branch is <code>main</code>. All pull requests should be made against this branch. If you need help getting started, [join us on Discord](https://documen.so/discord).
+
+1. [Fork](https://help.github.com/articles/fork-a-repo/) this repository to your
+   own GitHub account and then
+   [clone](https://help.github.com/articles/cloning-a-repository/) it to your local device.
+2. Create a new branch:
+
+- Create a new branch (include the issue id and something readable):
+
+  ```sh
+  git checkout -b feat/doc-999-somefeature-that-rocks
+  ```
+
+3. See the [Developer Setup](https://github.com/documenso/documenso/blob/main/README.md#developer-setup) for more setup details.
+
+## Building
+
+> **Note**
+> Please ensure you can make a full production build before pushing code or creating PRs.
+
+You can build the project with:
+
+```bash
+npm run build
+```