Docs: Update GettingStarted.md

[topisark]

Description

Update GettingStarted.md:

• Update Mapbox account links. The old links still work, but are not according to the new url schema.
• Remove legacy componentDidMount reference.
• Typo fixes and some text formatting.

Checklist

• [x] I've read CONTRIBUTING.md
• [ ] I updated the doc/other generated code with running yarn generate in the root folder
• [ ] I have tested the new feature on /example app.
  • [ ] In V11 mode/ios
  • [ ] In New Architecture mode/ios
  • [ ] In V11 mode/android
  • [ ] In New Architecture mode/android
• [ ] I added/updated a sample - if a new feature was implemented (/example)