Minor updates to alt text in pt/qgis-camadas and pt/geocodificando-qgis

[charlottejmc]

I am enlisting @joanacvp's help to cut down the alt-text for the two recently published PT translations, pt/licoes/qgis-camadas and pt/licoes/geocodificando-qgis.

Getting the right level of detail in the alt-text descriptions of our figure images is a difficult balance – we want them to be meaningful, but not too overwhelming or even frustrating for our users reading through a screen reader, which makes it difficult to skip through content.

I'll also take this opportunity to remove some stray captions from the EN-OR qgis-layers (some figure captions say "Click to see full-size image" despite no link allowing it).