Revision 2: Data Wrangling Cheat Sheet for DataFrames.jl v0.21

[tk3369]

This PR updates the Data Wrangling Cheat Sheet with revision 2.

Changes:

• Add Indexing Syntax section in the Select Variables block. Also mention that indexing syntax can select both observations and variables at the same time.
• Add mutation tips in the Select Observations block
• Add propertynames
• Compress cumulative/moving stats block
• Add ByRow example in Summarize Data block
• Add combine(f, df) and AsTable examples in Group Data Sets block
• Move Build Data Pipeline block to the left side

Targeted comments can be made at https://docs.google.com/presentation/d/1mpvPyR0z3MY23sPtzQP2xICBWU8XyGBuU2M2eyf4jPM/edit?usp=sharing

[bkamins]

Looks good. In the example ByRow is not strictly required but it is a correct syntax, so it is good.