Experiment: template enhancement output buffer

[mardelnet]

Summary

This PR experiments with the new template enhancement output buffer introduced with WordPress 6.9.

This output buffer is based on the hook wp_template_enhancement_output_buffer, which can be used to intercept the full HTML output of a template before being delivered to the browser. Among many other possibilities, this hook could help us to move some front functionalities to the server-side, improving their performance.

In this particular experiment, I use the wp_template_enhancement_output_buffer to replace the pdf_icon.js file, in its function of adding an icon to all the links to PDF documents.

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Testing

1. Add a link to a PDF document.
2. In the front, check that the link has an icon at the end.
3. Check that the icon is not included in links within headings (H1, H2,...) or links wrapping images.