[DOCS] update spreadsheet guidance

[stufraser1]

Updates to guidance on spreadsheet:

• Make this the start of Phase 2: Adding data to spreadsheet
• Phase 2 start : Add a note "any cell in light yellow is to be edited. If using a previously parameterized version the user should carefully review all entries to make sure they are correct."
• Define "average cost of payout per person" - explain this is used to determine a person affected trigger (the user may be confused because they may already have allowed for a per person cost in the loss simulator and thus feel they are double counting) - value to match in both tools.
• country loss analysis - remove reference to RPs
• add: why users would use yearly versus event loss
• add/move retention layer discussion to inital presnetation of layer 1/2
• solvency / 'how solvent' - explain further, link to exhaustion prob.

[stufraser1]

updated most in ac0e70e

@JPM-MaxInfo On the spreadsheet 'Country Loss Analysis', users can change the chart to show yearly loss or event loss. When yearly loss is shown, there is no aggregation of losses, and the only thing that changes is the label of the Y-axis. Is this as intended? I'd like to clarify so I can explain it and close this issue.

[JPM-MaxInfo]

It changes for me. I have made quite a few corrections to calcs though as I believe a column may have been added to country
input sheets and has thrown everything off fyi mine is in the develop-renv-install branch which I will look to shortly merge into develop