Update svg accessible name guidance

[khiga8]

1. Add a `<title>` inside the SVG, with the alt text/name.
2. Give the `<title>` an ID.
3. Apply `aria-labelledby` to the SVG, pointing to the ID of the `<title>` inside of it.

This approach has maximum compatibility and should be recommended above all other approaches. Not sure if we would necessarily want to flag the other approaches as errors though... 🤔

GitHub Staff-only link svg-alt-text