renamed modular content to linked items

[petrsvihlik]

Motivation

Based on our vision regarding components and content items, we have to rename "Modular content element" to "Linked items element". From the Delivery SDK's point of view, it means that we have to rename everything (methods/properties/classes/..., including SDK documentation) that contains "modularContent" to "linkedItems".

Keep in mind, that the Delivery API responses will remain the same. They'll still contain "modular_content".

Checklist

• [x] Code follows coding conventions held in this repo
• [x] Automated tests have been added
• [x] Tests are passing
• [x] Docu has been updated (if applicable)
• [x] Temporary settings (e.g. project ID used during development and testing) have been reverted to defaults

How to test

• there should be no reference to "modular content" in the SDK's API. instead, there should be "linked items" everywhere

[petrsvihlik]

@dusekdan @Simply007 could one of you please review it? thx