Fill expression functions docs with visual examples

The request is to add visual rendering of the functions (mainly the geometry ones) in the expression chapter. These can be gif or static images showing the expression and the output. For reference, changelogs sometimes provides good references for this. Some functions also have a corresponding algorithm we might have already provided an image for. Worth checking.

To add an image:

1. Fill the images_list.json file with necessary details (see template in first item)
2. Load the image(s) if not already in this repository, either in expressions/expression_help/img or processing_algs\*\img folder
3. (Optional) Execute the populate_expressions_list.py script if you locally have the code repo also. No worries if you do not do this, it will be handled by us.
4. Submit pull request

Page URL: https://docs.qgis.org/testing/en/docs/user_manual/expressions/functions_list.html#geometry-functions