Docs: Add link to API for GitHub imports

What does this MR do?

Adds link for the API to GitHub project imports to the main GitLab API page. Currently can only access this resource via a Google search.

Related issues

Pointed out by a customer in this ticket: https://gitlab.zendesk.com/agent/tickets/112763 (internal use only)

Closes

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards