📄 Footer: add 'About' page

This is the first attempt to provide an About page.

Composed with minimal text and structured with plain tags, they can always be enhanced as future work.

TODOs:

• Finalize endpoints for additional customizations
• Verify that all snippets work properly
• Verify spaces formatting (Jinja/HTML tags)
• Styling (including copy snippet button)

Future work:

• ToC (sticky on the left-hand side) for this page, seems quite long to read sequentially.
• Mention error codes for each endpoint (?)