Clean up and document poetry

[davidfarmer]

The poetry chapter of the Humanities in Action document https://pretextbook.org/examples/humanities/html/hia.html is not referenced in the main documentation, and that documentation does not even mention poetry.

In addition to expanding the documentation, also decide if @indent is better than preserving leading spaces in a line. Also note that neither is really correct because neither is guaranteed to give the desired appearance in a proportional font. Also, a directive like "set in italics" can be appropriate in the context of a poem, even though it is about presentation. In that case, em to get italics is tag abuse.

A problem with the current @indent is that the value has to be an integer, but the integer currently corresponds to 3 spaces. That is not sufficiently fine for certain cases.

See also #412 and #828