Update Contributing guide: Add more details to Debugging section

[bytrangle]

Summary
I hope to make the Debugging section of the Contributing guide clearer to newbie contributors. Here is what I add:

• Add more concrete instruction for configuring backend
• Explain what users should look for when changing dev-test/index.html
• Add more info to use local storage to set Netlify site's URL

Test plan

Checklist

Please add a x inside each checkbox:

• I have read the contribution guidelines.
• Code is formatted via running yarn format.
• Tests are passing via running yarn test.
• The status checks are successful (continuous integration). Those can be seen below.

A picture of a cute animal (not mandatory but encouraged)

[erezrokah]

Thanks @bytrangle! Sorry for the late reply, I was out of office.
I'm back now (!) and will be going through issues and the rest of the PRs

[bytrangle]

Thank you @erezrokah . Please don't say sorry. You've always done a splendid job tidying up the PR section.