docs: Add GitHub Adding Repositories page for onboarding help link

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Creates a focused page at /github/adding-repos to serve users who click the "Don't see the repos you need?" help link in the onboarding wizard. The page provides step-by-step instructions for managing repository access through GitHub organization settings.

Trigger Events

• Promptless/promptless PR #1820: onboarding wizard: handle edge cases for Slack install and GitHub repos

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[prithvi2206]

@Promptless instead of creating an entirely new article for this, is it possible to just have a redirect to the section of the gihtub integration page that talks about this?

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Good catch! I've already implemented the redirect approach on this PR. Instead of creating a separate article, the PR now adds a redirect from /github/adding-repos to the existing "Adding More Repos After Installation" section on the GitHub Integration page (#adding-more-repos-after-installation).

This way, users clicking the "Don't see the repos you need?" help link in the onboarding wizard will land directly on the comprehensive instructions that already exist, without any content duplication.