[DOCS] Add Documentation initial bin

[HeloiseS]

The initial bin in the HR Diagrams and the CMD is not the actual initial bin size. This is done to fudge the probabilities to appear correctly. Otherwise, the probability of the initial bin can take over the other bins due to its size. This information needs to be mentioned in the Documentation.

Possibly, it might also be necessary to look at the reason why the initial bin does still contain a large number of stars. (due to its size?)