docs: Added guidance on configuring Selenium within Home Assistant OS

[gareth305]

This pull request improves the guide for hosting a Selenium instance on a Home Assistant Supervised setup. The changes include:

• Clarified Instructions: Reworded and reorganized steps to ensure they are concise, clear, and easy to follow, especially for users unfamiliar with Docker or Portainer.
• Added Context: Explained key actions and decisions, such as why the seleniarm/standalone-chromium:latest image is used.

Why These Changes Are Necessary

• The original guide lacked structure and readability, which could lead to confusion or errors during setup.
• Clearer instructions help users complete the process efficiently, especially for those with limited technical expertise.
• Markdown formatting ensures the guide is compatible with various platforms, including GitHub, and aligns with documentation best practices.

[dp247]

This is definitely something that needs moving to the wiki at some point, but your changes do answer a question we get a lot