Update incident management workflow

Why is this change being made?

The current implementation of the Slack /incident command is soon to be replaced by Woodhouse: https://gitlab.com/gitlab-com/gl-infra/woodhouse.

• Remove screenshots and duplication of the incident declare Slack modal. This is doomed to fall out of date, and is no replacement for a self-explanatory UI.
• The previous documentation stated that the engineer on-call would be notified, but this isn't necessarily the case as they were not paged. Woodhouse optionally allows the engineer to be paged. Emphasise this choice.

A review of Woodhouse's incident management functionality from its staging deployment can be found here: https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11573

Issue to start using Woodhouse for incident management: https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11602

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.