img naming convention and organization

In response to Issue #125 a [translation workflow guide](https://github.com/AmericanRedCross/Guides/blob/master/TranslationWorkflow_LearnOSM/readme.md) is being developed.

This pull request contains edits of the English version with the intention of making the process of creating a new translation easier.

#### images:
- consistent use of reference style image syntax in chapters (previously a mix of reference and inline)
- language subfolder within images folder
- file naming convention:
	en_beg_[two digit chapter #]_
	[chapter name based on permalink]_
	image[two digit # code]_
	[image name based on image reference id]

**NOTE:** The chapter and image numbers are not a good system and hard to update. But for now, all the files in
"images\en\" are being used on the published pages (no old files not being used), the files are all in one place,
and the organization is consistent (making batch renaming easy with code or any number of utilities).

#### other (unrelated):
- link in *CONTRIBUTING.md* updated to skip a redirect