Add automated functional acceptance testing for Webshare with webship-js (Playwright + Cucumber-js), Drupal Canvas + Drupal CMS support, and MkDocs documentation

Problem/Motivation

• The share rail rendered through three different paths (block, Drupal Canvas, custom Twig) without a single source of truth,
  so improvements had to be made in multiple places.
• Enabling or disabling a platform did not invalidate the anonymous page cache — changes appeared stale until a manual cache
  rebuild.
• The settings form id contained a dot (webshare.config_form), producing a malformed HTML id and breaking
  automation that targets the form by selector.
• No automated coverage existed for Drupal CMS, where the Mercury theme delegates header and footer rendering to Drupal Canvas
  page_region entities instead of classic block layout.
• The module had no reference documentation for site builders, administrators, or developers.

Proposed resolution

• Single render path: The block, the Drupal Canvas component, and a custom Twig embed all render the same
  webshare:share Single-Directory Component.
• Drupal Canvas integration: The SDC is auto-exposed to Canvas as sdc.webshare.share and
  exercised on Canvas pages, content templates, and page-region entities.
• Drupal CMS support: A dedicated test site and feature suite for the drupal/cms distribution on
  the Mercury theme, with the share rail placed in mercury.header and mercury.footer Canvas
  page-regions.
• Cache invalidation fix: The rail now carries the webshare_platforms cache tag; saving the
  settings form or adding, editing, or deleting a platform invalidates it — every cached page that contains the rail is refreshed
  automatically.
• Settings form fix: The form id is now webshare_config_form and the rendered HTML id is the
  clean webshare-config-form.
• BDD test suite: 82 webship-js scenarios / 472 steps covering both flavours — smoke, front-end rendering,
  all 13 platforms, accessibility, responsive layouts, admin form, platform add/edit/delete modal, access control, Canvas, and
  Drupal CMS. Pure browser-driven steps backed by a 281-entry named-selector registry.
• CI: Two GitLab browser-test jobs (Drupal Standard + Drupal CMS) plus validate-stage jobs (cspell, eslint,
  stylelint) reproducible locally through gitlab-ci-local.
• Documentation site: MkDocs Material site under docs/ with Introduction, User, Administrator,
  Developer, and FAQ sections — builds cleanly under mkdocs build --strict.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Merge request, Patch, or Commit
• ✅ Testing to ensure no regression
• ✅ Automated unit testing coverage
• ✅ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ➖ Readability
• ➖ Accessibility
• ➖ Performance
• ➖ Security
• ➖ Documentation
• ✅ Code review by maintainers
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Release notes snippet
• ✅ Release webshare-2.0.0-alpha1

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• ci: #3591899 Add automated functional acceptance testing for Webshare with webship-js (Playwright + Cucumber-js), Drupal Canvas + Drupal CMS support, and MkDocs documentation