DOC-2254: Add Shadowing and Schema Registry context glossary terms

[Feediver1]

Description

Adds three glossary terms used by the new Confluent Schema Registry migration page in #1776:

• shadow link (Redpanda features)
• shadow cluster (Redpanda features)
• Schema Registry context (Redpanda features)

Related to https://redpandadata.atlassian.net/browse/DOC-2254

Note: DOC-1660 (Add Shadow Linking and DR terms to Glossary) is In Progress and may cover the two shadow terms. If that work already has definitions drafted, prefer those and drop the duplicates here.

Merge order: This PR should merge before or together with #1776, which uses glossterm:shadow link[] and glossterm:shadow cluster[].

🤖 Generated with Claude Code

[netlify]

✅ Deploy Preview for redpanda-docs-preview ready!

Name	Link
🔨 Latest commit	9aed0ed
🔍 Latest deploy log	https://app.netlify.com/projects/redpanda-docs-preview/deploys/6a46b8fe2e50ec0008a36a21
😎 Deploy Preview	https://deploy-preview-1777--redpanda-docs-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: b76d8348-f8c8-48e8-99a2-11ade65ba148

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch DOC-2254-shadowing-glossary-terms

---

_{Comment @coderabbitai help to get the list of available commands.}

[pgellert]

FYI, there are a few schema registry context-related definitions here: https://docs.redpanda.com/streaming/current/manage/schema-reg/schema-reg-contexts/#key-concepts

I am not sure what is worth documenting as glossary terms vs as concepts defined on the docs page, but I just wanted to highlight those in case you think they should be defined as glossary terms as well.