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.
It would be nice to explain to user that this statement downloads composer. And perhaps also link to composer docs to state they can just install composer globally and avoid doing it for each project.