Add valid data table and sample for editing data file in user guide

[tsulim]

Closes #252

As of now, there are no guide or sample on the format of the data file to follow, except for the generated data file during the first run of the application (which may be cleared by users). This will allow them to learn about the format and values required for amending the data file, reducing errors made in manually editing the data file.

Changes made:

• Add "Valid Data Values in persons" table
• Add "Valid Data Values in meetings" table
• Add "Sample Data File Content"
• Add note for Favourite and duplicates