doc: widget screens

### TimelineWidget

here be dragons

<img src="/img/widget_timeseries.png" width="600" />

### NumbersWidget

here be dragons

<img src="/img/widget_numbers.png" width="500" />

### BarsWidget

here be dragons

<img src="/img/widget_bars.png" width="600" />

### ToplistWidget

here be dragons

<img src="/img/widget_toplist.png" width="500" />

### HTMLWidget

here be dragons

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Complex Widgets

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There are a few high level widgets which have different semantics. FIXPAUL: explain...

### TimeSeriesGauge

here be dragons

- calculate-option: count, sum, progressive_sum, avg

- incr / incr_counter / incr_denominator

### DistributionGauge

here be dragons

### ToplistGauge

here be dragons

  → implement, _decr, _max, _min for zero config gauges

  → gauge explorer: fix gauges multiple times in select dropdown

  → doc: distribution, toplist and timeseries gauge