docs(connectors): document new GitLab operations

[wollefitz]

Description

Documents the following operations:

• Add Branches > List repository branches operation to the OOTB GitLab Connector connectors#2159
• Add Branches > Create repository branch operation to the OOTB GitLab Connector connectors#2160
• Add Repository files > Create new file in repository operation to the OOTB GitLab Connector connectors#2161
• Add Merge requests > Create MR operation to the OOTB GitLab Connector connectors#2162

When should this change go live?

• This change is not yet live and should not be merged until {ADD_DATE} (apply hold label or convert to draft PR)?
• There is no urgency with this change.
• This change or page is part of a marketing blog, conference talk, or something else on a schedule.
• This functionality is already available but undocumented.
• This is a bug fix or security concern.

PR Checklist

• I have added changes to the relevant /versioned_docs directory, or they are not for an already released version.
• I have added changes to the main /docs directory (aka /next/), or they are not for future versions.
• My changes require an Engineering review, and I've assigned an engineering manager or tech lead as a reviewer, or my changes do not require an Engineering review.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer, or my changes do not require a technical writer review.

[ev-codes]

@christinaausley this PR looks good to me!

May you please double-check and approve if this looks ok to you?

[christinaausley]

@wollefitz is this only for the next version of docs, or also the current 8.4?

[wollefitz]

@wollefitz is this only for the next version of docs, or also the current 8.4?

This is only for next.