[DOCUMENTATION] Make public keys of releases available

[Bibo-Joshi]

What kind of feature are you missing? Where do you notice a shortcoming of PTB?

We sign our releases but currently do not provide the public keys so users are not able to verify those signatures.

Describe the solution you'd like

Publish the public keys both on

• The PTB github repo, i.e. put them into a directory and point there from the readme
• the homepage python-telegram-bot.org. Maybe on a new sub-page that we can link on the start-page.

Additionally the "how to release" wiki page should be updated and tell maintainers to add their public keys in these spots as well.

Describe alternatives you've considered

No response

Additional context

No response

[Bibo-Joshi]

Closed by #3436 and python-telegram-bot/python-telegram-bot.github.io@4249eb9