Add example content to experimental install profile

Problem/Motivation

This issue picks up on the idea started quite a long time ago here: #79582: Add example content to the experimental out-of-the-box demo install profile, with some differences and somewhat broader scope. :)

Where that issue suggested giving users a better experience in the default profile, the proposal here is to include example content in the experimental profile proposed in #2809635: Create experimental installation profile.

Among other things this avoids arguments that arise out of the "this is the face of Drupal" assumption. It also allows us to join forces with the team working on #2759849: Create a new user-facing core theme as part of a Drupal demo and provide a finite set of content that they can theme to so that they don't need to cover every possible use case as themes like Bartik attempt to do.

Proposed resolution

Use the 'farmers market' scenario from our documentation to create the content model of an experimental install profile:

You are making a website for a farmers market. The site needs to display information about the location and hours of the market, and an About page with the history of the market. It also needs to list the vendors. Vendors should be able to edit their listings (including a logo or photo), and post recipes. Site visitors should be able to browse recipes, or locate recipes using ingredients that they purchased at the market. Some visitors to your site speak another language, so the main pages and vendor pages need to be translated.

And populate that profile with example content.

There are three key benefits to this approach.

1. It gets us past the endless debates over what the content should be
2. It resolves the issue of 'is it real world or instructive content'. Because the tasks in documentation use this example we solve both problems at once
3. Because the user guide content was designed to demonstrate common concepts in Drupal, the model is already geared towards showcasing all of the varieties of content re-use Drupal excels at

Remaining tasks

1. Make sure the "experimental profile idea is accepted
2. Agree on the actual interpretation of the model outlined in the guide
3. Set up a sandbox configured with that model
4. Write example content and populate the sandbox
5. Get feedback from the 'new theme' project #2759849: Create a new user-facing core theme as part of a Drupal demo
6. Iterate until it looks great

User interface changes

1. There will be a new profile in core labelled something like "Core plus example content (experimental)".
2. After install the user will see the example content.

API changes

None

Data model changes

None