docs: add baseline documentation
accept: add add_paragraph.feature
api: add Document() factory function
api: Document loads default template
tmpl: add default.docx
api: Document raises on not .docx

docx.api.Document should raise on construction if supplied file is not
a Word .docx file.
api: add _Document.body
oxml: add docx.oxml.base module
parts: integrate _Document with opc part factory
broken: refactor _Document.load() to new signature
parts: add _Document.body
parts: add _Body.add_paragraph()
oxml: add CT_P.new()
oxml: add CT_Body.add_p()
text: add Paragraph.add_run()
oxml: add CT_R.new()
oxml: add CT_P.add_r()
text: add Run.add_text()
oxml: add CT_Text.new()
oxml: add CT_R.add_t()