fixed some urls in guide; moved elasticsearch png file to proper location, to be seen by apidoc generator

Didn’t touch the end yet, with the TBD et al.

Updated extension docs to be less restrictive about choosing namespace, more neutral composer.json example

Make clear that "type" in extensions' namespaces must be in plural form (widgets/behaviors/modules)

updated database, based on @cebe's comment https://github.com/yiisoft/yii2/pull/1485#discussion_r8410809

worked though all the guide files adjusted structure and
added some missing does