Improve "Compiling with script encryption" instructions to make compilation requirement clearer

**Your Godot version:**
3.x

**Issue description:**
Users who try to use script encryption often miss critical requirements from this page that they need to compile Godot export templates from source.

The page should be improved to make this much more prominent, link to relevant pages on how to compile Godot, explain why the encryption key is set in an environment variable, how to make sure that it has indeed been included in the compiled template, how to use said template, etc.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/development/compiling/compiling_with_script_encryption_key.html