docs: update beta.md with copyedit for clarity

[OlliesGudh]

This PR rewrites our Beta Testing page as brought up to discussion on my topic yesterday. The goal was to bring updated information and to make it clearer and more engaging for readers.

Changes include:

• Simplified sentence structure for better readability and flow
• Several minor tweaks on phrasing to make it more friendly/international
• Updated tone to make our beta program more inviting, highlighting benefits in a community-centered way
• Updated instructions on how to find the beta program to match the new Play Store app listing layout

Overall, the page now contains updated information, reads more naturally and gives users a clearer idea of how they can help test AntennaPod, while keeping the friendly and open tone consistent with the rest of our documentation.

edit: typo

[ByteHamster]

I think we should also add something along the lines of "just using the beta doesn't really help. You also need to give feedback". For almost every release, people only report bugs after we roll it out to stable.

[OlliesGudh]

Would you like me to work on that? Any suggestions on text structure? May I propose a version here on the comment section for review before any new commit?

[ByteHamster]

Any opinions @keunes?