Add glossary term for leader pinning

[kbatuigas]

Description

This pull request adds a new documentation partial for the "leader pinning" feature in Redpanda. It introduces a brief definition, hover text, and a reference link for further information.

Documentation update:

• Added a new partial leader-pinning.adoc describing the "leader pinning" feature, including its purpose, a hover-text explanation, and a link to detailed documentation.

Resolves https://redpandadata.atlassian.net/browse/
Review deadline:

Page previews

Checks

• New feature
• Content gap
• Support Follow-up
• Small fix (typos, links, copyedits, etc)

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[Feediver1]

lgtm