docs: add feature analysis page for sp-solid-fill
acpt: rename feature files to reflect prs taxonomy
acpt: rename step files to remove step_ prefix
acpt: add change-shape-fill-color feature
shp: refactor TestShape to DescribeShape

* rename AutoShapeType._lookup_id_by_prst() to .from_prst()
* change AutoShapeType staticmethods to class methods
* change Shape._adjustments to @lazyproperty
* others ...
oxml: standardize XML data builder

* move BaseBuilder to tests.unitdata
* standardize organization of unitdata modules, builder classes on top
in alpha order, factory functions at the bottom, no docstring for
factor functions
test: refactor test_autoshape.py to pytest (step 1)
test: refactor test_autoshape.py to pytest (step 2)
test: refactor test_autoshape.py to pytest (step 3)
shp: rename AutoShapeType.id_from_prst()

from AutoShapeType.from_prst(prst), as it simply returns an autoshape
type id, not an AutoShapeType instance.
shp: update test_autoshape.py to BaseBuilder

Change XML builder calls in test_autoshape.py for BaseBuilder
subclasses in unitdata.autoshape.py replacing first-get XML builder.
oxml: refactor oxml.test_autoshape to BaseBuilder
shp: add Shape.fill
dml: add FillFormat.solid()
spike: interim commit

just after adding:

* it_knows_its_brightness_adjustment
* it_can_set_itself_to_an_RGB_color
* it_raises_on_assign_non_RGBColor_type_to_rgb
reorg: split dml into .color and .fill
dml: add _ColorFormat.theme_color setter

along the way:

* factored enumeration lookup over to oxml module
dml: add _ColorFormat.brightness setter
dml: complete _ColorFormat test coverage
acpt: add theme color round-trip scenario