Clarify the order of operations

[FossPrime]

Mistakes I made, admittedly low on sleep:

• Used root dispite all warnings
• Ran non-prebuilt code... and couldn't undo.

Clarified by using headings and re-ordering to make prebuilt section stand-alone

[rom1v]

Thank you.

I moved more prebuilt server-related doc and changed minor things, here is the result: fc5de88

Is it ok for you?

Then I wrote a script to simplify installation (on install_release branch):
https://github.com/Genymobile/scrcpy/blob/install_release/BUILD.md#simple

I will merge both on master soon if everything is ok.

[FossPrime]

Looks good! I was concerned about breaking the #Prebuilt-server link, but making people read from top to bottom is probably desireable.

[rom1v]

I was concerned about breaking the #Prebuilt-server link

Yes, me too, but Github-markdown does not allow to force an anchor name (without breaking other things) 😕

But anyway, with the "simple" method, few people would need to use the prebuilt server explicitly (they'll just use install_release.sh I guess).