[Feature] Additional source suggestions

## Problem

More search engines are always welcome in a project like this, to improve the number of relevant options to users.

## Description

While we're in the [process of renaming](https://github.com/WordPress/openverse-frontend/issues/1197) it to 'External Sources', since the meaning of 'meta search' is a bit confusing for folks (not to mention, using the word 'meta' in a project with '-verse' in its name seems ill-advised), a feature similar to this search engine is still quite popular in Openverse.

Our list of sources might be a good reference for some to include in _this_ meta search. We also have the guidelines for how to format their URLs:

https://github.com/WordPress/openverse-frontend/blob/main/src/utils/get-additional-sources.ts#L61

## Description

## Alternatives

## Additional context

## Implementation

- [ ] I would be interested in implementing this feature.