Update readme

[zoglo]

Disclaimer No, this is not just some tutorial PR from some YouTuber to learn using GitHub, I wanted to test the current RC and went to the readme for quick instructions installing the demo :D. Seen that it still doesn't have the LTS branch within the readme (5.2, not 5.3) and I kinda went all out after fixing that one thing

Description

• Updates the current working branch to 5.3 within the readme (was 5.2)
• Adds a table of contents
• Adds alternative texts for images
• Adds some badges
• Added code formatting to composer installation so we can copy those commands
• Added link to the current demo
• (Added .gitattributes so composer ignores the newly added picture within the /docs folder and it doesn't get downloaded into the vendor folder)

I don't think we need to merge this downstream as 5.3 is the main branch presented on GitHub and packagist

TL;DR: Dark mode readme

Light mode readme

Live-example can be seen here: https://github.com/zoglo/contao-demo/tree/update/readme

[aschempp]

I like! @sascha-mueller should we merge this?