Add a note on new repos needing a commit for resync pickup

[phalbert]

Description

This PR adds documentation clarifying GitLab repository sync behavior, specifically addressing the limitations around new repository detection and initial commits.

Changes

• Added an info callout explaining GitLab system hook webhook limitations
• Clarified that new repositories require an initial commit to trigger webhook events
• Added information about resync behavior for empty repositories
• Reorganized the system webhooks section for better readability

Context

This documentation update was prompted by this slack thread about repository sync behavior in the GitLab Ocean integration. Specifically:

• Empty repositories (those without commits) are only detected during scheduled resyncs
• GitLab doesn't provide webhook events for project creation
• A repository needs at least one commit to trigger webhook events

References

• GitLab Webhook Events Documentation
• Related support thread discussing repository sync behavior

Updated docs pages

Please also include the path for the updated docs

• GitLab Installation (docs/build-your-software-catalog/sync-data-to-catalog/git/gitlab/installation.md)

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