doc: refactor _Body.add_paragraph()

* Migrate (text=‘’, style=None) interface for add_paragraph() down to
actual implementation so it can be shared when extracted to
BlockItemContainer.
* Combine unit tests for add_paragraph() in test_api.py
blkct: introduce BlockItemContainer

* base _Body on BlockItemContainer
* no substantial inheritence yet, just getting it wired up
oxml: remove dead code from CT_Body

* also fix type in Length class docstring
blkct: add BlockItemContainer.add_paragraph()

* remove _Body.add_paragraph(), inheriting that method from
BlockItemContainer
blkct: add BlockItemContainer.add_table()

* remove _Body.add_table(), inheriting that method from
BlockItemContainer
* refactor Describe_Body.it_can_add_a_table() to use cxml instead of
XML builders
* retain test for _Body.add_table() to test integration with required
oxml elements.
blkct: add BlockItemContainer.paragraphs

* remove implementation from _Body to allow inheritance
blkct: add BlockItemContainer.tables

* remove implementation from _Body to allow inheritance
tbl: base _Cell on BlockItemContainer

* use super() on _Cell.paragraphs rather than inherit so a custom
docstring can be provided.
tbl: add _Cell.add_paragraph()

* use super() rather than direct inheritance to allow custom docstring
tbl: add _Cell.add_table()
tbl: add _Cell.tables
opc: add content type for .docx package

Needed to set Content-Type header in an HTTP Response object carrying
a .docx payload.
release: prepare v0.7.4 release