`sheet-by-name` not documented

[shriram]

https://www.pyret.org/docs/latest/tables.html

The table method sheet-by-name is not documented. Not even clear what other methods there might be (because simply evaluating a table shows the content, rather than listing the fields/methods).

[blerner]

@jpolitz I've written a scaffolding of docs for this, so we're mostly done with it. Todo:

• we don't have scribble formatting for documenting fields of an object (unlike data constructors), and
• when I compile it locally, I get different formatting than the current pyret-docs site. Not sure if this is a scribble version issue, though :-\

[blerner]

The commit above (and its sequels) have been merged. Marking as horizon-fixed, because it will be, shortly.