Add CC0 license and README project overview

[afoote-mitre]

Summary

Add the supplied CC0 1.0 Universal license to the CVE Search API repository and make it easy to find from the README. The README also gains a brief project overview and a table of contents for the existing documentation.

Changes and Rationale

Change	Primary File	Why It Was Needed
Add the supplied CC0 1.0 Universal license	LICENSE	Makes the license available directly in the repository without modifying its text.
Add a project introduction, license link, and table of contents	README.md	Explains the API's role, identifies the license, and improves navigation while preserving the existing operational guidance.

This PR is limited to LICENSE and README.md. The previously proposed code of conduct, contribution guide, pull request template, and security policy are excluded. No application code, dependencies, or runtime behavior changes.