Transform: Updated documentation

[ajdapretnar]

Updated documentation for:

• Data Sampler
• Select Columns
• Select Rows
• Transpose
• Merge Data
• Select By Data Index
• Aggregate Columns
• Group by
• Apply Domain
• Impute
• Continuize
• Randomize
• Purge Domain
• Formula
• Create Class
• Python Script

Workflows added for reproducibility.

[markotoplak]

/home/runner/work/orange3-doc-visual-programming/orange3-doc-visual-programming/source/widgets/data/purgedomain.md:: WARNING: image file not readable: widgets/data/images/PurgeDomain-Example.png

[janezd]

I fixed a typo and non-consecutive header levels, though these will appear everywhere. Scaling of images is also random.

Both issues are rather technical and we should tackle them in one sweep. We'll need to do the scaling systematically via some plugins or whatever.