Document page block magnification feature

[tjerman]

About

Page block magnification allows users to focus/expand specific page blocks over the entire page.

The contents should be placed in a new file in the compose-configuration section.

The contents should:

• Outline what the feature is, what it can be used for, and some use-cases,
• Outline how to enable/configure the feature,
• Outline what the different page block magnifications do (provide screenshots on how they look like).

Original issue https://github.com/cortezaproject/corteza/issues/450

Important

Any screenshots should follow our guidelines -- from taking the screenshot to annotating it.

[katrinDY]

For today: add remaining documentation and submit for code review

[katrinDY]

for today: address comments from PR and re-submit for review