GitLab Duo Chat: Provide /summarize command for issue, epics, MR summaries

Release notes

Problem to solve

I kept using /summarize in GitLab Duo Chat but it is not officially implemented yet. I added it to a docs MR and learned about its state. !145875 (comment 1792622123)

Proposal

Implement, support and document /summarize as a Chat command.

It should take the opened website into context, for example an open issue/epic/MR.

Intended users

Feature Usage Metrics

Does this feature require an audit event?

/cc @tlinz

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