Clarify GitHub status checks permissions and visibility

[bruno-garcia]

Some organizations do not see GitHub status checks because their Sentry GitHub App installation does not include the checks:write permission. This is currently not obvious to users.

From the thread:

• Status checks are not feature-flagged; they depend on GitHub app permissions.
• If the app installation does not grant checks:write, status checks will not appear.
• There is a suggestion to, at a minimum, document the status checks feature and its permission requirements.
• There is also a suggestion to improve the GitHub integration UX to:
  • Explain during onboarding why specific permissions (like checks:write) are requested.
  • After onboarding, make it easy in the product to see the current integration state, which permissions are missing, and what functionality is affected (e.g. clearly flag when status checks for reviews will not show because checks:write is not granted).

Scope of work:

• Add documentation for the GitHub status checks feature, including the need for checks:write and how it impacts visibility of status checks.
• In the GitHub integration UI, consider surfacing the granted/missing permissions and clearly indicating when status checks will not be available due to missing permissions.

[linear[bot]]

NET-544

[jamescrosswell]

@bruno-garcia how/where do people interact with Sentry's github integration from the .NET SDK? I don't really get what we need to do here...