Update svg accessible name guidance

```
1. Add a `` 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](https://github.com/github/accessibility/pull/2273/files)

Review the repository's current SVG accessible-name guidance and compare it with the linked GitHub accessibility reference. Decide how the title, ID, and aria-labelledby pattern should be recommended and how alternative approaches should be treated; done means the guidance reflects that decision.

索引モデルが issue の本文から書いたものです。