Broken link for OpenAPI Sandbox

### Problem

API docs link to the OpenApi Sandbox which was removed in #13402

https://openlibrary.org/developers/api
https://openlibrary.org/swagger/docs

### Reproducing the bug

1. Go to https://openlibrary.org/developers/api
2. Click the "OpenAPI Sandbox" button

* Expected behavior: Explanation that the page has been removed, button removed to reflect that
* Actual behavior: Page 404s

### Context

- Browser (Chrome, Safari, Firefox, etc): N/A
- OS (Windows, Mac, etc): N/A
- Logged in (Y/N): Y
- Environment (prod, dev, local): prod

### Breakdown

Implementation Details (for maintainers)

#### Requirements Checklist
* [ ]

#### Related files

*

#### Stakeholders

*

#### Instructions for Contributors

- Please [run these commands](https://docs.openlibrary.org/developers/tools/git.html#working-on-your-branch) to ensure your repository is up to date **before** [creating a new branch](https://docs.openlibrary.org/developers/tools/git.html#making-changes-and-creating-a-pull-request) to work on this issue and **each time after** pushing code to Github, because the pre-commit bot may add commits to your PRs upstream.

Start by locating the source for the API documentation at openlibrary.org/developers/api and the OpenAPI Sandbox link, then review issue #13402 for the removal context. Remove or update the button and ensure the page explains the Sandbox has been removed; verify that the link no longer leads to the 404 page.

Written by the indexing model from the issue text.