updates to new redoc link

[ramadanomar]

Fixes

#900 by @ramadanomar . If i find any more dead/expired links , i'll try to update them here to prevent from filling out the pr/issue list with small changes like this.

[AetherUnbound]

This actually points to an image which is referenced on the produced documentation page here: https://wordpress.github.io/openverse-api/guides/quickstart.html#steps

The image itself can be found here: https://github.com/WordPress/openverse-api/blob/main/api/docs/_static/api_redoc.png

While reviewing this though, I did notice that the API's README references the frontend dev docs rather than the API!

openverse-api/api/README.md

Line 5 in 5fae4eb

- [Developer docs](https://wordpress.github.io/openverse-frontend/)

This PR could adjust that link instead 😄

[krysal]

@ramadanomar Could you keep working on this PR? The linked issue is fixed but it would be nice to correct the image that @AetherUnbound mentioned. It's okay if you are not available at the moment, we understand, just (preferably) let us know :)