Adds Operations Modes section

• Modifies the index page to add a new Operations Modes section;
• Moves the financial and programming mode documents within the Operations mode section and removes the financial calculations section;
• Renames the financial page to mode-financial to maintain a simple naming convention;
• Adds screenshots of the programming and financial modes to the respective mode pages;
• Adds an overview page listing the operation modes of the calculator;
• Renames financial.page to mode-financial.page, add mode-programming and mode-overview pages to meson.

Ref: #185