docs: restructure docs and add tutorials (#2883)

Co-authored-by: knqyf263 <knqyf263@gmail.com>

docs/tutorials/overview.md

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+# Tutorials
+
+Tutorials are a great way to learn about use cases and integrations. We highly encourage community members to share their Trivy use cases with us in the documentation.
+
+There are two ways to contributor to the tutorials section
+
+1. If you are creating any external content on Trivy, we would love to have it as part of our list of [external community resources][community-resources]
+2. If you are creating an end-to-end tutorial on a specific Trivy use-case, we would love to feature it in our tutorial section. Read below how you can contribute tutorials to the docs.
+
+## Process for adding new tutorials
+
+Requirements
+- The tutorial has to provide an end-to-end set of instructions
+- Ideally, tutorials should focus on a specific use case
+- If the tutorial is featuring other tools, those should be open source, too
+- Make sure to describe the expected outcome after each instruction
+
+**Tip:** Make sure that your tutorial is concise about a specific use case or integration. 
+
+How to add a tutorial
+
+1. Simply create a new `.md` file in the tutorials folder of the docs
+2. Add your content 
+3. Create a new index in the mkdocs.yaml file which is in the [root directory](https://github.com/aquasecurity/trivy) of the repository
+4. Create a PR
+
+[community-resources]: additional-resources/community.md