Add configurable editor instructions field with help page for creating accessible PDFs

Problem/Motivation

Editors uploading PDFs have no guidance within the module on how to create accessible PDFs. There is also no
way for site administrators to customize instructional text shown to editors.

Proposed resolution

• Add a configurable "Editor instructions" field to the settings form, with a default message linking to
  the help page.
• Implement hook_help() with a detailed, configurable help page covering PDF accessibility
  steps.
• Add a "Help page content" textarea to the settings form so administrators can customize the help
  page.
• Add pdfa11y.help config object with schema for storing help content.
• Add an update hook to set defaults for existing installations.

Remaining tasks

• Review and commit.

User interface changes

• New "Editor instructions" textarea on the settings form.
• New "Help page content" collapsible details section on the settings form.
• New help page at /admin/help/pdfa11y.

API changes

None.

Data model changes

• New editor_instructions string added to pdfa11y.settings config.
• New pdfa11y.help config object with help_content string.