[customer feedback] Add related examples to "filter" functionality on geoJson source (style spec)

[sentry-io]

Sentry Issue: DOCS-FEEDBACK-4AN

"filter" functionality on geoJson sources needs examples to be useful - I spent a while assuming that it would work in the same fashion as the layers for filtering geometry-type, it's not clear from *this* particular page how flexible it can be.

• page: https://docs.mapbox.com/mapbox-gl-js/style-spec/sources/
• site: Mapbox GL JS
• section: undefined
• helpful: False

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[palhal]

I agree. One would assume that this works:
filter: ['==', ['geometry-type'], 'Point']

But it doesn't. I ended up including the geometry type as a property, resulting in this workaround:
filter: ['==', ['get', 'type'], 'Point']