Implemented scalable grouped-question pages for prototype_v4_2.

View prototype: https://lung-health-check-pr-78.herokuapp.com/

Changes include:

• Added pages.yaml as the source for page-level content and composition:
  • page heading and optional caption
  • description/details
  • ordered list of question IDs
  • conditional question display via if, includes, excludes, any, and all
• Kept questions.yaml focused on reusable question controls:
  • type
  • answerKey
  • input.label
  • hints, options, validation, errors and summary labels
• Added support for caption-only page headings, so a page can use a question label as the H1 while setting its own caption.
• Added grouped-page infrastructure:
  • page loader in lib/question-pages.js
  • renderQuestionPage()
  • validateQuestions()
  • shared _question-input.html partial
  • grouped _question-page.html template
• Added and wired grouped pages:
  • /about-you
  • /asbestos
  • /smoking-duration
  • /tobacco-smoking
  • /tobacco-smoking-change
• Updated flow so:
  • weight pages continue to /about-you
  • /about-you continues to respiratory conditions
  • respiratory conditions continue to grouped /asbestos
  • smoking-duration replaces age-started-smoking, age-stopped-smoking, and periods-stopped-smoking
  • tobacco-smoking replaces separate frequency and quantity pages
  • tobacco-smoking-change replaces separate change frequency, quantity, and years pages
  • shisha follows the same tobacco-smoking flow but skips the smoking-change flow
• Removed unused standalone pages, routes and controllers for:
  • asbestos at work/home pages, now combined into /asbestos
  • cancer history
  • individual smoking duration pages
  • individual tobacco frequency/quantity/change pages
  • old shisha smoking-setting flow and related summary/seed/docs content
• Updated check-your-answers links so grouped answers change via their grouped pages:
  • gender, sex, ethnicity, education -> /about-you
  • asbestos answers -> /asbestos
  • smoking duration answers -> /smoking-duration
  • tobacco smoking answers -> grouped tobacco pages
• Cleaned up YAML by moving labels into input.label, removing empty input: {} blocks, and reducing avoidable quoted scalar values.
• Updated docs and index links:
  • question-schema.md
  • question-flow.md
  • tobacco docs
  • prototype page index