Improve and explain migration of media from standard WP install to playground

[Lovor01]

This issue is also a comment in trac #60375

I was having problem with setting playground demo of my plugin to work - more precisely, images did not show. I solved it by creating zip file with uploads folder and modifying exported WXR file to replace url's to reflect local file system. I also understand that it could be done in another way, by exporting database, replacing urls in database and importing it into playground.

As a temporary solution to ease media transfer I propose following:

1. Add property "rootImportURL" in blueprint json importWXR and runSql steps - if this property exists in these steps, URL-s would be modified as a part of import step in a way that root URL is replaced by new installation root URL. In playground, root URL is "/". So, for example https://mydomain.com/ would be replaced by /
2. Create documentation which explains steps how to incorporate media files in installation - I propose writing docs for current situation and afterwards they would be replaced by this easier solution which was proposed in 1.

[bgrgicak]

@Lovor01 we are trying to collect examples like this in the Blueprint gallery.
It would be great if you would contribute your example there.