Clean up the `dsp` branch

[jouni]

The dsp branch has several issues:

• The "Foundation" section has duplicate pages with almost identical content:
• "Figma Libraries" page is duplicated (one page at the top level, another under "Components").
• The articles/ds/ folder is unnecessary, now that the script that picks updates from the latest branch can move pages. We should remove the extra ds/ folder.
• The articles/styling/ folder is not shown on the website at all, since it’s outside the ARTICLES_PATH folder. This breaks the "Styling" card link in the "Components" section index page, and links from the individual component "Styling" sub-pages. I wonder if it’s even beneficial to add this section to new DSP projects.
• The scripts/dspublisher-init.js file should be removed after the project has been initialized.
• None of the component pages should be marked as "commercial" (the purple dot after the navigation item is not visible, but the extra class name is there from the section-nav front matter attribute).
• The dspublisher/docs-theme/header.ts file is deprecated API. It should be dspublisher/docs-theme/init-browser.ts instead. But I’m not sure there’s much benefit in giving users empty files in their project. Same with the dspublisher/docs-theme/init.js file.
• The [discussion-id] line should be removed from all content pages.
• A lot of broken cross references in the pages. Some are likely due to the duplicate content in the "Foundation" section.
• A missing include file (looks like this part of the pick script is not working https://github.com/vaadin/docs/blob/latest/scripts/pick/cherry-pick-config.js#L108-L111):

  error /Users/jouni/Downloads/dsp-test/myds-docs/articles/ds/components/grid/flow.asciidoc, line 36 - include file not found: /Users/jouni/Downloads/dsp-test/myds-docs/articles/ds/binding-data/_items-identities.adoc
  

• All pass:[<!-- vale ... -->] lines should be removed
• What should we do with components behind feature flags? Should those be included, and how should they be documented then?