Improve onboarding val

[pmillspaugh]

Work-in-progress: https://val.town/x/petermillspaugh/hello-val

Rationale:

• Lean into simplification, progressive disclosure
• README focus less on explaining what Val Town is (the user already signed up) and instead focus on what a val is. Remove mention of "triggers" because users can grok it by going through the files (or by following links to docs)
• Weather is a fun example, but switch to GitHub for more useful/"serious"
• Hono is awesome, but switch to Web standards so that new users unfamiliar with Hono don't need to learn another tool (but link to docs on JSX/Hono)
• Sprinkle in a few example vals for each trigger to get wheels turning
  • [ ] TODO: brainstorm the coolest vals to place there!
• HTTP example could be expanded to something more practical (e.g. receive Stripe webhook). On the simple side for now
• GitHub for two of the examples because it's so universal and a very easy API to work with. Could use GitHub for every file as a shared theme, but it might be better to spread bets across different examples