Update writeFiles documentation

[bph]

the step is listened under Blueprints > Steps > WriteFiles

https://wordpress.github.io/wordpress-playground/blueprints/steps#WriteFilesStep

filesTree (DirectoryResource) – The data to write
writeToPath (string) – The path of the file to write to

"resource" for filesTree with possible values 'literal:directoryorgit:directory`
"name" - the name of the root directory
"files" - an object where:
Keys are filenames/paths
Values are either: String: content for files
Nested objects for subdirectories

I would also add a representation of the directory structure

my-plugin/
├ index.php
├ public
├─ style.css