[UX] Improve discoverability of option to change hero image

[stpaultim]

Description of the need

In office hours some people asked about how to add a hero image to the font page of the site. When we demonstrated the easy ability to add an image to the default "hero block" on the home page layout, they were quite impressed and expressed frustration that this was not easier to discover.

The problem may be the following:

There is a default image on the home page, but when visiting the settings for that block - it appears empty. This could easily leave the impression that this is NOT the place to change the default image on the front page.

A change to version 1.30 will add some help text that may improve this situation. We have added some help text to the hero block form AND we have added a preview of the hero image, if one has been added (by default, this field will remain empty).

So, while already implemented changes may improve this situation already, other ideas have been suggested.

Proposed solution

1. Adding default content to the hero block that helps make this more transparent.

2. Adding this to the "Get Started" suggestions on dashboard.

3. Improving this text.

4. Adding a default image/photo on installation.

This would need to be done in a way that does not break contributed themes and has been discussed extensively here:
#6752

Alternatives that have been considered

Option 4 has gotten the most discussion. An initial PR to test this option broke many contributed themes and has been ruled out.

[indigoxela]

I think each of the (wording) suggestions is helpful. And I do belief, the combination will actually fix the "WTF is this thing" when looking at the home page layout right after install - for newbies and also for people familiar with D7 but not yet familiar with Backdrop (or not familiar with D7 panels). 👍

Some wording suggestion: "Add your own background image by editing this 'hero block' and uploading your custom image to it." (Just a quick suggestion, I'll leave it to native speakers to improve.)

Could we please drop suggestion 4 from the issue description?
We already discovered that it's impossible to do that without breaking contrib, so why does this suggestion keep on showing up again and again? Impossible is and stays impossible. And I stated several times now (and will continue), that this doesn't even address the problem.