Add explanations for basic concepts

As a reader I want to understand the basic concepts of the solidbase app by reading https://learn.solidbase.info/en/solidbase/

Explanations of basic concepts and techniques should go in the Introduction:

• activities
• expenses
• expense category texts
• income
• privateness of budget data
• decimal setting
• super admin status

Some of them need also better explanation in 3. Create new (empty) budget as @leos reported:

? Could be more described the link mentioned by “can view your budget using a link” (for “Private option)”? ? What does “use decimals for amounts” option do in the “Edit meta data of Budget” window? Could be added description? I see decimal regardless the setting.