feat: add Eleventy shortcode

[ascorbic]

Related to #624

Introduces the @unpic/11ty package, providing an Eleventy shortcode for embedding responsive images using the unpic-img library.

• Package Creation: Adds a new package @unpic/11ty with dependencies on @unpic/core and @11ty/eleventy. Includes a package.json and a main index.js file implementing the unpic shortcode.
• Shortcode Implementation: Implements the unpic Eleventy shortcode in index.js, utilizing @unpic/core to transform image properties and generate an <img> tag with responsive attributes.
• Documentation and Examples: Adds a README.md in the @unpic/11ty package detailing installation and usage instructions. Also, updates the main README.md to include Eleventy in the list of supported frameworks.
• Demo and Testing: Includes an Eleventy demo site in examples/11ty demonstrating the use of the unpic shortcode. Modifies the .github/workflows/e2e.yml to add Eleventy to the end-to-end tests.
• Documentation Update: Updates the documentation site with a new page docs/src/content/img/11ty.md explaining how to use the @unpic/11ty package in Eleventy projects.

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