Fix accessibility issues in the pattern library UI and demo pages

[shyusu4]

Description

Fixes accessibility issues, across two areas:

The pattern library UI

• Heading order (Accessibility issue: Heading levels should only increase by one on /django-pattern-library/demo/ #279)
• Landmark uniqueness (Accessibility issue: Landmarks should have a unique role or role/label/title (i.e. accessible name) combination on /django-pattern-library/demo/ #280)

The demo/test example pages

• Heading order (Accessibility issue: Heading levels should only increase by one on /django-pattern-library/demo/render-pattern/patterns/pages/people/person_page.html #301, Accessibility issue: Heading levels should only increase by one on /django-pattern-library/demo/render-pattern/patterns/pages/search/search.html #303, Accessibility issue: Heading levels should only increase by one on /django-pattern-library/demo/render-pattern/patterns/pages/test_page/test_page.html #309, Accessibility issue: Heading levels should only increase by one on /django-pattern-library/demo/render-pattern/patterns_jinja/pages/search/search.html #322, Accessibility issue: Heading levels should only increase by one on /django-pattern-library/demo/render-pattern/patterns_jinja/pages/test_page/test_page.html #326)
• Empty heading (Accessibility issue: Headings should not be empty on /django-pattern-library/demo/render-pattern/patterns/pages/test_page/test_page.html #308)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Fixes #279, Fixes #280, Fixes #301, Fixes #303, Fixes #308, Fixes #309, Fixes #322, Fixes #326