Document /goal in GitLab Duo Agentic Chat

What does this MR do?

Adds a "Work toward a goal" section to the GitLab Duo Agentic Chat docs for the /goal command in the GitLab UI.

The section covers:

• What /goal does: it starts a Developer Flow session that keeps working toward the goal, instead of answering one turn at a time.
• Prerequisites: Chat open in a project, and the Developer Flow prerequisites, including turning the Developer Flow on for the top-level group.
• The two ways to start a goal (/goal or More actions > Goal) and how to remove the goal before sending.

UI strings match the frontend on master (prompt_input_actions.vue, goal_token.vue, plugins/goal/index.js).

Before merging

• Confirm duo_chat_goal_command is enabled by default (default_enabled: true) or removed in 19.5, so /goal is available on GitLab.com by default, SM can be enabled via the feature flag. On master today it's still wip and default_enabled: false.
• Confirm 19.5 as the introduced milestone.

Related issues

• Epic: gitlab-org#23107
• Release note: !260813 (closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the documentation process and style guide.
• Assign the relevant technical writer for review.