Update messaging docs title to include appends

[mschristensen]

Description

Updates the documentation title and navigation label for the messages updates and deletes page to reflect that it also covers message appends functionality.

Changes made:

• Updated navigation menu item from "Updates and deletes" to "Updates, deletes and appends" in src/data/nav/pubsub.ts
• Updated page title from "Updating and deleting messages" to "Updates, deletes and appends" in src/pages/docs/messages/updates-deletes.mdx

This change improves discoverability and accurately represents the full scope of message mutation operations covered in this documentation section.

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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