Update TTIG documentation

[LDannijs]

Summary

Update the documentation of the TTIG so that the steps are clearer.

Why do we need this ?

I had trouble onboarding the TTIG in the beginning, and going through the forum, other people have some issues with the clarity of it as well, eg: https://www.thethingsnetwork.org/forum/t/cant-get-my-gateway-connected-blink-red-and-green-forever/62315

What is already there? What do you see now?

...

What is missing? What do you want to see?

...

How do you propose to document this?

I have some experience writing good documentation (Starter kit) so ill be able to update it.

Can you do this yourself and submit a Pull Request?

Yes, @KrishnaIyer