[HeaderStackView] Remove color theming docs and mark the color themer as to be deprecated.

[jverkoey]

No description provided.

[codecov-io]

Codecov Report

Merging #3612 into develop will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #3612      +/-   ##
===========================================
+ Coverage    54.85%   54.85%   +<.01%     
===========================================
  Files          320      320              
  Lines        27305    27258      -47     
===========================================
- Hits         14978    14953      -25     
+ Misses       12327    12305      -22

Impacted Files	Coverage Δ
...ents/Snackbar/src/private/MDCSnackbarOverlayView.m	61.07% <0%> (-2.8%)	⬇️
components/TextFields/src/MDCTextField.m	77.97% <0%> (-1.78%)	⬇️
components/NavigationBar/src/MDCNavigationBar.h	90% <0%> (-0.91%)	⬇️
...exibleHeader/src/MDCFlexibleHeaderViewController.m	73.43% <0%> (-0.79%)	⬇️
components/Ink/src/private/MDCLegacyInkLayer.m	36.91% <0%> (-0.74%)	⬇️
components/Snackbar/src/MDCSnackbarMessageView.m	50.77% <0%> (-0.71%)	⬇️
components/Slider/src/MDCSlider.m	78.54% <0%> (-0.34%)	⬇️
components/Chips/src/MDCChipField.m	16.62% <0%> (-0.27%)	⬇️
...ponents/FlexibleHeader/src/MDCFlexibleHeaderView.m	69.94% <0%> (-0.04%)	⬇️
...onents/TextFields/src/MDCTextInputControllerBase.m	89.69% <0%> (-0.01%)	⬇️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b3fcaf1...d68ba01. Read the comment docs.

[jverkoey]

Merging without review because it is only documentation.