Add Openverse API source to documentation in sources.md

[Goziee-git]

Fixes

• Fixes Update sources.md to include Openverse data source information #229 by @Goziee-git

Description

This PR updates the sources.md file with the relevant information about Openverse as a datasource in the project and its API

Changes

sources.md: Added Openverse section with API endpoints, supported media types, and license options
• Documented ~240 result limitation per source-license combination
• Noted approximate counts due to anonymous access restrictions

Details

• 700+ million works from 20+ sources
• Supports images and audio media types
• All Creative Commons licenses and public domain
• No API key required for basic access

Checklist

• I have read and understood the Developer Certificate of Origin (DCO), below, which covers the contents of this pull request (PR).
• My pull request doesn't include code or content generated with AI.
• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

Developer Certificate of Origin

For the purposes of this DCO, "license" is equivalent to "license or public domain dedication," and "open source license" is equivalent to "open content license or public domain dedication."

Developer Certificate of Origin

Developer Certificate of Origin
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Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.


Developer's Certificate of Origin 1.1

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(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
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[TimidRobot]

Good work, thank you!