docs: provide information about how an indicator is calculated and what the result value is

[matthiasschaub]

[matthiasschaub]

In the dashboard it could look like: hiding more general information on how indicators and especially labels/classes are calculated behind the colored flag on the top-left of a indicator result in the dashboard

[matthiasschaub]

For Currentness indicator:

    out_of_date ≥ 30 % → class 1
    up_to_date ≥ 75 % → class 5
    up_to_date ≥ 50 % → class 4
    up_to_date+in_between ≥ 75 % → class 3
    up_to_date+in_between ≥ 50% -> class 2
    → class 1


class 1: red
class 2, 3: yellow
class 4, 5: green