Document possible values for webhook target type

[jothilakshmij]

Added possible values for X-GitHub-Hook-Installation-Target-Type header.

Why:

The documentation for the X-GitHub-Hook-Installation-Target-Type webhook delivery header previously described its purpose but did not specify the possible values it may contain. This lack of detail made it difficult for developers to reliably distinguish between repository, organization, and GitHub App webhooks using header information alone.

This change improves clarity and completeness for developers implementing webhook handlers.

Closes: #41739

What's being changed (included screenshots):

Expanded the documentation for the X-GitHub-Hook-Installation-Target-Type header in the Delivery headers section. Added a structured table listing the currently used values: *repository *organization *app

Included concise descriptions for each value to clearly indicate the webhook source.

Added a short explanatory note describing how this header can be used to distinguish webhook origins without relying solely on the payload structure.

This update is documentation-only and does not change any existing webhook behavior.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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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
webhooks/webhook-events-and-payloads.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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