Reorder extensions files and rename to be extension structure to be clear its more than just files its directory strcutre and improve doc a bit

Two recent SE questions have made me think that readers are missing this
key set of installing CiviCRM with civibuild (instead of with the normal
installation steps). I've added clarification here which I hope will
prevent future users from stumbling on this step.

ref:

https://civicrm.stackexchange.com/questions/19294/how-to-create-entity-in-civicrm
https://civicrm.stackexchange.com/questions/19194/unable-to-locate-folder-xml-under-the-civicrm-root-directory

This way MkDocs will be able to report errors for broken links

Done with:

perl -0777 -i -pe \
's@(?<!!)\[([^\]]*)\]\((?!https?://)(?!mailto:)(/(?:(?:(?:/(?![\)#]))|[^/)#])(?!.md))+)/?(#[^)]*)?\)@[$1]($2.md$3)@gm' \
`find . -name '*.md'`

Close #86

Updated line 627 to "For use with CiviCRM 4.3 and later," per info from @totten

http://www.writethedocs.org/guide/writing/docs-principles/#cumulative
Creating a new entity should come before creating a database upgrader

remove hookref-old.md and move all content into relevant files. Add wordpress setup information for hooks.

"Request a review" instead of "request a request".

(files still need to be moved around)

1) This seems like a site admin task, not a developer task. If you're reading this page, I assume you
   already know how to do this. I'd argue this belongs in the user guide (if it's not already there).
2) It doesn't seem right to me that one should configure their extensions directory before
   deciding whether or not to write an extension. First check the extensions directory to
   see if someone has already built the thing. If not, only then does make sense to start
   playing with your environment config.
3) If we keep it in the developer guide, I'd put it at the beginning of the instructions
   for creating a module extension (https://wiki.civicrm.org/confluence/display/CRMDOC/Create+a+Module+Extension).
   These are currently undergoing migration from the wiki to mkdocs, so I'm blocked from making such
   an update at this time.