Add support for SVG icons

It would be nice to be able to import and inline SVG files via a shortcode.

It seems reasonable to not need to meaningfully parse or modify the actual file (e.g. we can assume it includes things like `fill="currentColor"` internally on relevant path components), but it would be nice to include a CSS `icon` class with properties like `width:24px;height:24px;display:inline`.

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For context, I'm wanting to include things like menu item icons in the headings of the corresponding documentation. It may work to do that via an iconify plugin for tailwind (instead of specifically handling custom SVG files and including the relevant individual files in the relevant docs repo), but I'm not sure whether they would then not be the colour of the surrounding text, which would be a problem, and I don't love the idea of importing thousands of icons when only a few are needed. That would also be insufficient if the softwares are using any custom icons that we want to reproduce in the docs.

No file or test is named in the issue. Start by locating the theme's existing shortcode handling and how its styles and static assets are included; done means a selected SVG can be inlined without modifying it, receives the requested icon styling, and renders correctly in documentation headings.

Written by the indexing model from the issue text.