Editing the pdfjs-viewer docs

/pdfjs-viewer/ is one Markdown page with a live preview of the viewer beside it.

Prose: src/pdfjs-viewer/index.md. Headings ## and ### make up the table of contents.
Attribute tables and playground controls: src/_data/pdfjsAttributes.js.
Version numbers: read at build time from the installed @missing-elements/pdfjs-viewer. The docs serve that package's dist/ as the live preview, so bumping the dependency updates both.

Try buttons

Add try after the language of a code fence to give it a ▶ Try button that applies the snippet to the preview:

```html try
<pdfjs-viewer-element src="/file.pdf" zoom="page-width"></pdfjs-viewer-element>
```

HTML snippets copy the attributes of their <pdfjs-viewer-element> onto the preview. style, id, class and the placeholder src="/file.pdf" are skipped.
JavaScript snippets run as written. document.querySelector('pdfjs-viewer-element') returns the preview element.
Sample addresses: in both kinds, the first https://missing-elements.github.io in an attribute's value, or in a script, is replaced by the origin the docs run on, so that a local preview plays its own samples (localUrl in src/pdfjs-viewer/docs.js).
Every Try button comes with a Reset button that puts the preview back to its defaults.
try=fresh replaces the preview with a new element first. Use it for settings that only apply when the viewer is built, such as setViewerOptions and the "before connect" attributes.