Document page icon feature

[katrinDY]

Description

The page icon feature allows us to set an icon to a page. This gives the user the ability to further customize a page. An icon can be uploaded (.ico, .png, .jpg) or a link can be provided

Documentation should go here

• [x] Compress images
• [x] Document how to enable the feature (if any) and how to use it
• [x] Consider providing an example

Original issue https://github.com/cortezaproject/corteza-docs/issues/368

[katrinDY]

For today: add description to issue and start working on it

[katrinDY]

For today: continue working on this and submit for review

[katrinDY]

For today: address comments and resubmit for code review

[katrinDY]

For today: address comments and resubmit for code review

[katrinDY]

For today: address comments and resubmit for code review