Docs: port articles from wiki: REST api, Smart app model, Portlet, WebDav

[tusmester]

Port these articles from wiki, because they are referenced from new docs here on github:

• [x] http://wiki.sensenet.com/Smart_Application_Model
• [x] http://wiki.sensenet.com/OData_REST_API
• [x] http://wiki.sensenet.com/Portlet
• [x] http://wiki.sensenet.com/Webdav
• [ ] Update the links here in the install-sn-from-nuget.md and sensenet-components.md files.

[borsi]

Should we also move the images to our sn-resources repository?

[tusmester]

We agreed on storing images for docs locally in the repo where the doc is created, in the docs/images subfolder.