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[].

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Preview pages

This PR changes term partials only, so the rendered surface is the glossary page:

• Glossary (updated — all three new terms verified rendering in this preview: shadow link, shadow cluster, Schema Registry context)

Note: this shared-branch preview assembles the 25.3 build; the shared branch feeds every version, so the terms will appear in all versions' glossaries once merged.

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[micheleRP]

@Feediver1 Suggest removing the "For more info" on each glossterm. We removed on all other entries earlier because of problems with links, especially with older versions.

[Feediver1]

PR Review: Add Shadowing and Schema Registry context glossary terms (#1777)

Files reviewed: 3 glossary term partials (base: shared)
Overall assessment: Clean, accurate definitions that follow glossary conventions — including the no-links rule, now that Michele's feedback is applied in d9f9124. One useful discovery: the overlapping ticket DOC-1660 is effectively orphaned, which resolves the PR body's open question.

Michele's feedback — applied (d9f9124)

Removed the "For more information" xref line from all three partials. The rationale verified against the corpus: none of the other 133 glossary entries carries an xref: link — the single remaining link (admin-api.adoc) is a version-independent link:/api/doc/admin[] URL, which is exactly the versioned-xref breakage that motivated the rule.

Verification performed

• No duplicate term files (shadow* / *context* sweep across all 134 partials — only these three plus the unrelated context-window.adoc).
• Consumer usage matches exactly: #1776/#1780 use glossterm:shadow link[] and glossterm:shadow cluster[] with case-sensitive term-name matches.
• The shadow-link definition's replication list matches the Shadowing overview, correctly omitting 26.2-only role sync since the shared branch serves all versions.
• Category and casing conventions match existing entries.

DOC-1660 overlap — resolved, needs Jira triage

The PR body defers to DOC-1660 (Shadow Linking glossary terms) if its definitions are further along. They aren't going anywhere: DOC-1660 is In Review but assigned to an inactive account (Angela Simms has left). Its definitions are engineering-internal (ShadowPanda, mirror partitions) and consistent with this PR's user-facing ones — no conflict. This PR should proceed; DOC-1660 needs reassignment or closing with these two terms counted as partial delivery.

Suggestion

No consumer currently uses glossterm:Schema Registry context[] (#1776 uses plain prose in the contexts section). The term is still worth having, but consider wiring #1776's first mention to it — that was the point of adding it.

Merge order

As the PR body states: before or with #1776, whose glossterm: macros resolve against these terms.

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[Feediver1]

@micheleRP applied in d9f9124 — all three "For more information" lines removed. Confirmed the convention while at it: the other 133 glossary entries have no xref links (admin-api's version-independent link:/api/... is the lone, deliberate exception), so the hover-text definitions now stand alone as intended.

[micheleRP]

lgtm