Add "Checks" permissions to github-app docs

[arogge]

Description

the documented permissions were incomplete. To make the plugin actually work, you have to also enable Read/Write permissions on Checks.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Automated tests have been added to exercise the changes
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verify that the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given

Documentation changes

• Link to jenkins.io PR, or an explanation for why no doc changes are needed

Users/aliases to notify

[jglick]

To make the plugin actually work, you have to also enable Read/Write permissions on Checks.

Well, technically this plugin does not do anything with Checks, only commit statuses. But you would likely want to combine it with github-checks, which as the name implies does require write permission on Checks. (It is also possible to use another plugin to disable the commit status generated by this plugin, so that you do not actually need commit status permission, if you care.) Anyway, it seems sensible to guide users to enable the most commonly required permissions even if not always necessary.

[jglick]

Best to make that explicit, and also update https://plugins.jenkins.io/github-checks/#plugin-content-features correspondingly.

[jglick]

see #746