Replace templated demo descriptions

At least 16 demo todo pages carry the identical description: 'Create a demo page for the X component.'. That field is what Meta.svelte emits as <meta name="description">, so search engines see duplicate descriptions across the site.

Affected pages

demo-anchor-heading, demo-articles, demo-authors, demo-call-to-action, demo-call-to-action-black, demo-img, demo-link-block, demo-meta, demo-post-preview, demo-publication-authors, demo-publication-date, demo-read-next, demo-reading-time, demo-share-post-link, demo-sitemap, demo-todo-cover, demo-accordion.

Tasks

• Write a unique 1-2 sentence description for each demo describing what the component actually demonstrates
• Also revisit the templated cta.description stubs ('Articles showcase', etc.)