Document page block auto refresh feature

[tjerman]

Page block auto refresh lets configurators pull and show new data when the content behind it changes.

The documentation should go here

• [x] Document how to enable it and what configuration options are available
• [x] Document what page blocks support it and any plans to support the ones which are currently not
• [ ] Provide an example

Original issue https://github.com/cortezaproject/corteza/issues/466

[tjerman]

Will get a PR going for me in a bit

[katrinDY]

For today: finish task

[katrinDY]

for today: address comments from PR and resubmit for code review

[EmmyMay]

For today: continue to address the review comments.

[katrinDY]

For today: address comments and resubmit for review