Rewrite power poses block as "preparing for a presentation"

https://github.com/CodeYourFuture/CYF-PD/blob/main/content/blocks/public-speaking-circles-of-influence/readme.md has a tab on power poses, which relates to some prep we're no longer doing (https://github.com/CodeYourFuture/Module-JS2/issues/43#issuecomment-1733179161)

It looks like it maybe was meant to have been deleted, but got missed?

But I think replacing it with a similar-length block about preparing for giving a presentation is worthwhile.

Ideally I think this would mostly be an opportunity to get the class to think about what works for them and share ideas (because different people will be motivated by different things), but I imagine we could discuss some things like:
1. Breathing control
2. Feeling well prepared (i.e. "If you have thought about / written your talk, you'll be less stressed than if you just wing it")
3. Mentally framing yourself (e.g. reminding yourself how much you _do_ know about a topic, rather than dwelling on how much you don't)