Docs feedback: Enhance Github integration documentation
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<!-- /label gitlab-ce~9444928 - Correction or clarification needed. -->
<!-- /label gitlab-ce~9444930 - Improving an existing doc; e.g. adding a diagram, adding or rewording text, resolving redundancies, cross-linking, etc. -->
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### Problem to solve
Need to create a documentation that's helps to use github service API.
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### Further details
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this page: https://docs.gitlab.com/ee/api/services.html contains a bunch of service documentation and could be the right place to hold the github service documentation
### Who can address the issue
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Github service contributors ?
### Other links/references
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https://gitlab.com/gitlab-org/gitlab-ce/issues/34848
issue