Create docs for text handling/rendering

[rich-iannone]

Documentation is currently a bit limited w.r.t. text input for a wide variety of methods (e.g., cols_label(), tab_header(), etc.).

We seem to be missing:

• term for text handling (e.g., md(), html(), inline units)
• term for inline units
• explanations for where these can be used (could start with a simple list of methods and expand as necessary)

We ought to have a section in the Get Started guide that generally explains the value of inline units in a table (e.g., why use inline units at all?).

[machow]

One more piece from recent discussions, it might help to have a one sentence explanation os what a "unit" is, and what "units" are (e.g. is everything inside the {{}} shorthand a unit? or is it "units" and a unit is a smaller piece)