DOCS-39: update github collector reference

[jeff-matthews]

Summary

Update all reference docs for the OpenHound GitHub collector based on the upstream changes (SpecterOps/openhound-github#55):

• Schema
• Nodes
• Edges
• Privilege Zone rules
• Saved searches

Replacement PR for #377

The upstream schema doesn't define icons for some nodes, so the following images are missing when running doc validation:

opengraph/extensions/github/schema.mdx
 ⎿  /images/extensions/github/gh_runner.png
 ⎿  /images/extensions/github/gh_runnergroup.png
 ⎿  /images/extensions/github/gh_secret.png
 ⎿  /images/extensions/github/gh_variable.png

It's not a blocker, but the schema should probably be updated with icon definitions.

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