feat(cookbook): add mock data and formulas for market share chart walkthrough

[jdhoffa]

Enhance the market share illustration section in the cookbook with simple mock data and corresponding calculations. This will support client understanding of how formulas translate into chart lines and make the methodology more accessible.

Tasks

• Create a mock loanbook containing 3 ICE auto manufacturers with example production values.
• Create a mock ABCD containing 10 auto manufacturers, including the 3 portfolio companies, with example production values.
• WSalk through the formula for SDS line (target trajectory)
• Walk through the formula for the Portfolio line
• Walk through the formula for the Corporate Economy line

Acceptance Criteria:

• Tasks above completed
• Each formula is shown with mock data substituted and the resulting values are explicitly calculated.
• Three lines are derived from the formulas: SDS, Portfolio, Corporate Economy.
• Content is self-contained and understandable without needing external references.
• Content is reproducible
• Can be easily reused or referenced in further client-facing documentation.
• Ensure tables and formulas are visually aligned with the style of current (r2dii.plot) charts.