Add explainer for forecast report

[OlegWock]

I'm not native english speaker, so if there any mistakes in the explainer feel free to correct me

Text in the modal:

This report uses Monte Carlo Simulation to calculate the probability of having a different net worth in 10 years.

Based on your prior net worth change history, it projects 100 possible scenarios in which your net worth could change in the next 10 years. It then calculates the minimal net worth for the top 90%, 75%, 50%, 25%, and 10% projections (i.e., in the top 10% of projections, your final net worth was $xx,xxx or more).

For each bucket, an example projection is then selected and displayed on chart.

GitHub Issue (if applicable): #3323

Explanation of Bugfix/Feature/Modification:
Added button which shows info how forecast report works

Screenshots

[joshmadewell]

seeeeeeeeeeeeeems good to me!