Add guidance on when to use GitHub Apps

[ahmasmibhanu]

Summary
Adds a new section to the GitHub Apps overview explaining when to use GitHub Apps.

Details
This change introduces a short “When to use GitHub Apps” section to help readers understand common use cases, such as deep integrations, fine-grained permissions, and security benefits like short-lived tokens. The content aligns with existing guidance and improves clarity for users deciding between GitHub Apps and other integration options.

Type of change

Documentation improvement (no functional changes)

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
apps/overview.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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