Use plone.distribution and add voltodemo with testdata

[pbauer]

Replaces #17

This is a mayor rewrite of the demo-sites that uses plone.distribution

For Volto:

• Add distribution voltodemo in the package plone6demo
• Use volto-light-theme and the following add-ons: volto-form-block, @eeacms/volto-accordion-block, @kitconcept/volto-button-block, @kitconcept/volto-heading-block, @kitconcept/volto-introduction-block, @kitconcept/volto-highlight-block, @kitconcept/volto-separator-block, @kitconcept/volto-slider-block
• Create new demo-content, partly imported from the light-theme-demo site.

For Classic:

• Add distribution classicdemo in the package plonedemo.site
• Add collective.easyform and example forms
• The content is mostly the same as the old demo-site but now created with plone.distribution

This depends on collective.exportimport==1.12 and plone.distribution==1.0.0b3.

These were required to make volto-sites portable:

• Fix migrating bocks  collective/collective.exportimport#241
• Fix importing blocks on the root plone.distribution#56

The Volto frontpage currently looks like this:

[erral]

I think that using volto-light-theme for the official Plone demo site is wrong. I think that the site should look like the standard Plone installation: Volto with its default theme.

Do not get me wrong. I find volto-light-theme totally OK and impressive, we are even trying to customize it a bit for a client's project, but until this is not the official theme for Volto it shouldn't be used in the Plone official demo installation.

Moreover taking into account that it also installs several additional addons. It has its own demo site, which is OK.

I am also in favor of linking it from the demo site pointing to an "example site that uses volto with a custom theme" or whatever text.

[stevepiercy]

We have two links to the official demo sites in documentation. I'm fine with more.

https://6.docs.plone.org/install/index.html#try-a-plone-demo

[pbauer]

We discussed that at the sprint after the Plone Tagung and at the Volto Team Meeting and I got the impression that everyone was happy to showcase a more realistic demo-site enhanced with add-ons and the light-theme.
I agree that a "pure" demo has its merits but it does not do all that much to help market Plone. Maybe having both is good solution.
With the exportimport feature of plone.distributions it will be much easier to maintain both or even more of these for different use-cases, with different demo-content and different.

I suggest we first fix the deployment of the demo-sites (#36) and then create a pure demo (that still can finally have demo-content). The classic-site can stay as it is because we only added easyform.

[stevepiercy]

I think we need both the pure and VLT demos.

It is great to compare an out-of-the-box Volto site that you install locally against the pure demo. We use this a lot when filing issues of how to reproduce UI issues. We need to keep it.

It is great to show off features that come with Plone with a few popular add-ons as in this demo for marketing purposes. The pure demo is OK, but it is never what I would deploy for a client. I want to showcase something and say, "Hey, client, look at this awesome site, login, and edit its content, and tell me that you still want to Wix/WordPress/Weebly/OtherShittyWebsiteEditor without throwing up in your mouth a little."

I would wholeheartedly support adding a link to Try a Plone demo. Pull request gratefully accepted!