Add default gatherer replacement doc example

**Summary**

As seen in https://github.com/dequelabs/axe-core/issues/4259 there are a number of important accessibility APIs that `axe-core` does not currently, and may not for a while, cover, like `role`s applies via `elementInternals`. The means that it is easy on an element that is give a `role` in this manner to receive this output:
```
❌ `aria-prohibited-attr` failure for `minScore` assertion (Elements use prohibited ARIA attributes: https://dequeuniversity.com/rules/axe/4.9/aria-prohibited-attr )
Expected >= 0.9, but found 0
```
Other than moving `aria-prohibited-attr` at large to a `"warn"` is there more nuanced configuartion that can be applied to prevent this false negative? For instances, some integrations of `axe-core` allow for `ignoredTags` to be listed or `ignoredRules`. For my cases, I think I really need a `ignoredRulesByTag` sort of thing, which I'm not sure is actually possible at the `axe-core` level, but am seeing what's possible.