docs: add AI SDK glossary

[Karunasagar12]

Summary

• Adds an AI SDK Core glossary page for shared terminology
• Defines initial terms from the issue, including language model call, model, provider, provider specification, tools, streams, and callbacks
• Links the glossary from the AI SDK Core index card list

Fixes #14234

Test Plan

• Verified the new MDX file is UTF-8 readable and includes frontmatter
• Verified the AI SDK Core index page is UTF-8 readable after adding the glossary card