Fix/update download

Description

Applies a new structure to download section, updates requirements and allowed branch names for MRs

Testing Instructions

1. [Step 1: use mkdocs serve to see website]
2. [Step 2: have a look at the download sections an merge requests]

Developer Checklist

• Code has been tested locally and/or in pipeline.
• Merge conflicts resolved with the target branch.

docs/download/getting-started.md

-You want to **use** UNICADO to get familiar with the workflow and see first results? Great :fire: Then check out the [Installation Guide](installation.md) and the [Cleared for Take-Off](takeoff.md). It includes the prerequisites, troubleshooting hints and a standalone installer.
+You want to **use** UNICADO to get familiar with the workflow and see first results? Great :fire: Then check out the [Requirements](requirements.md) for installation and usage. Afterwards, you can download and install UNICADO [here](takeoff.md).
 
-If you're a **developer** interested in contributing to UNICADO, follow the [Developer Installation Guide](../get-involved/developer-installation.md). However, you also need to check out the [Installation Requirements](installation.md) first!
+If you're a **developer** interested in contributing to UNICADO, follow the [Developer Installation Guide](../get-involved/developer-installation.md). However, you also need to check out the [Requirements](requirements.md) to ensure you'll be able to run your code!