task: #3627251 Use our own body and summary fields in the default content

Issue: https://www.drupal.org/project/webapi_starter/issues/3627251

Plan: 03-own-body-field. Webpage, Web Blog, Web Doc and Web Releases move from core's body to their own field_body (long text) and field_summary (plain text) fields (webpage !22, webblog !16, webdoc !13, webreleases !19). This MR moves the default content and the template's references to them.

• Default content: body → field_body (value and format unchanged) and field_summary (the summary, when the node had one; an empty summary is filled on save from the first paragraph).
• Checked with a script: every value, format and summary is the same as before, and nothing else in the content files changed.
• Display Builder sources and views that pointed at body now point at field_body, and blog teasers at field_summary.
• Tested on Drupal 11.4.8 in DDEV with the four module branches: fresh site:install of the template, 0 watchdog errors, front, login and admin pages 200, the blog teasers show the summaries.
• Merge after the four module MRs; release together with them.

AI-Generated: Yes

Checkpoints

• File an issue about this project
• Addition/Change/Update/Fix to this project
• Testing to ensure no regression
• Automated unit/functional testing coverage
• Developer Documentation support on feature change/addition
• User Guide Documentation support on feature change/addition
• UX/UI designer responsibilities
• Accessibility and Readability
• Reviewed by a human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Update Release Notes
• Release