[Docs]: Update and improve the Discord weather bot tutorial about shuttle

[jonaro00]

@Lonanche All yours! Have fun :D

[c-git]

I skimmed through it and it looks good. I didn't actually try it like I planned because I want to use poise for my project as I expect it may grow and that is recommended in the tutorial. Thank you very much for updating this.

[Lonanche]

I would like to have a go on this

[Lonanche]

@jonaro00

List of changes I made:
-Updated tutorial code snippets to use Serenity 0.12 (shuttle-hq/shuttle-docs#264)
-Changed one screenshot because discord changed how to obtain discord guild ID
-Submitted final code of tutorial to serenity/weather-forecast in shuttle-examples (shuttle-hq/shuttle-examples#146)
-Made some small changes in tutorial because some parts were unclear while I was following it

Let me know if any of the stuff have to be changed!

[c-git]

@Lonanche happy you got to this. I was planning to try to do it tonight. So I'll give yours a test run and give you feedback instead.