reorg: shorten 'Collection' suffix to 's'
api: eliminate Document constructor function
docs: add shapes page to user guide
acpt: add api-add-paragraph.feature
api: add Document.add_paragraph()

Along the way:

* move DocumentPart.get_or_add_image_part() into proper sort order
doc: add DocumentPart.add_paragraph()
api: add Document.paragraphs
doc: add DocumentPart.paragraphs
text: add Paragraph.text
acpt: add api-add-heading.feature
api: add Document.add_heading()
api: add Document.add_picture()
acpt: add api-add-page-break.feature
api: add Document.add_page_break()
acpt: add api-add-table.feature
api: add Document.add_table()
api: add Document.tables
doc: add DocumentPart.tables
acpt: add tbl-add-row-or-col.feature
tbl: change Table.rows.add() to Table.add_row()