Declare incident via /incident declare command in Slack

Why is this change being made?

As part of IMA (Incident Management Automation) work, we have a new way to declare incident from within Slack. This work is documented in this issue: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/9162. Having an outdated incident declaration information is not right because it would confused engineer on calls. Therefore, making sure we have the most update to date information captured.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what