docs: document run content feature analysis
acpt: add par-insert-paragraph.feature

Acceptance test for Paragraph.insert_paragraph_before() API method.
doc: small refactoring on Document.add_paragraph()

Document.add_paragraph() can rely on Paragraph.add_run() to add the
text.
doc: refactor some names

‘p’ has generally come to be reserved for a <w:p> element and
‘paragraph’ used for a Paragraph instance. Similaryly ‘r’ indicates a
<w:r> element and ‘run’ an instance of Run.
para: add Paragraph.insert_paragraph_before()