3.0 minor corrections

- use `src/` convention against `/src/` to refer to top level folders (src, config, log, tmp, webroot, tests). Linked to #1622
- update comment on DEBUG non-zero level to refers to `true` or `false` instead
- add an anchor to `App.baseURL` so the end of Url rewriting page could work properly : http://book.cakephp.org/3.0/en/installation/url-rewriting.html#i-don-t-can-t-use-url-rewriting
- small updates on Blog Tutorial
- update and order folders after installation.
"a simple" :P