docs: Fix doc missing step

[emi-dm]

This pull request updates the docs/quickstart.md documentation to improve guidance for users implementing projects with the tool. The main changes clarify the workflow by introducing a new task definition step, and provide tips on phased implementation for complex projects.

Workflow improvements:

• Added a missing step for generating an actionable task list using the /speckit.tasks command, clarifying the process between checklist validation and technical planning.
• Updated step headings and order to reflect the new workflow: defining tasks precedes generating the technical plan, and validation/implementation is now a later step.

Guidance on phased implementation:

• Added a tip recommending phased implementation for complex projects to avoid overwhelming the agent's context, both in the general workflow and in the Taskify example.

[Copilot]

Pull request overview

This pull request updates the quickstart documentation to add a missing workflow step and provide guidance on phased implementation for complex projects. The changes aim to clarify the development process by explicitly separating task definition from technical planning.

Changes:

• Added two tips recommending phased implementation to avoid overwhelming the agent's context
• Introduced a new Step 5 in the detailed example for generating tasks with /speckit.tasks
• Renumbered subsequent steps in the detailed example (former Step 5→6, former Step 6→7)

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