Consolidate release notes for 3.6

[QuLogic]

PR Summary

Also add a couple of missing documentation entries for the new pages.

PR Checklist

Tests and Styling

• [n/a] Has pytest style unit tests (and pytest passes).
• [n/a] Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [n/a] New features are documented, with examples if plot related.
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).

[jklymak]

Sorry, this was my wording, but if we can clarify:

Suggested change

	``tight_layout`` and ``constrained_layout`` are incompatible if a colorbar has
	been added to the figure. Invoking the incompatible layout engine used to warn,
	but now raises with a `RuntimeError`.
	You cannot switch between ``tight_layout`` and ``constrained_layout`` if a colorbar has
	already been added to a figure. Invoking the incompatible layout engine used to warn,
	but now raises with a `RuntimeError`.

[jklymak]

Suggested change

	Axes title now avoids y-axis offset
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Previously, Axes titles could overlap the y-axis offset text, which is often in
	the upper left corner of the axes. Now titles are moved above the offset text
	if overlapping, and automatic positioning is in effect (i.e. if *y* in
	`.Axes.set_title` is *None* and :rc:`axes.titley` is also *None*).
	Axes title now avoids y-axis offset
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Previously, Axes titles could overlap the y-axis offset text, which is often in
	the upper left corner of the axes. Now titles are moved above the offset text
	if overlapping when automatic title positioning is in effect (i.e. if *y* in
	`.Axes.set_title` is *None* and :rc:`axes.titley` is also *None*).

[QuLogic]

Some additional PRs:

• ENH: Add dark/light mode theme to the buttons #22607
• Add text.parse_math rcParams #22556
• ENH: add ability to remove layout engine #22452
• Add a registry for color sequences #22387
• ENH: compressed layout #22289 (above)
• ENH: Use rcParams savefig.directory on macosx backend #22180
• Change colour of Tk toolbar icons on dark backgrounds #22163
• Use standard toolbar in wx. #22013 maybe?
• DOC: remove experimental tag from CL #21975
• FIX: Update blitting and drawing on the macosx backend #21790
• Set macOS icon when using Qt backend #21784
• [ENH]: Use new style format strings for colorbar ticks #21542
• MNT: Add modifier key press handling to macosx backend #21512
• ENH: Add support to save images in WebP format #21274
• ENH: Add full-screen toggle to the macosx backend #22936
• ENH: Add option to disable raising the window for macosx #22928
• negative_linestyles kwarg in contour.py #23266
• Standardize edge-on axis locations when viewing primary 3d axis planes #23644 maybe?
• Add markerfacealt to pass-through arguments for error bar lines #23475
• FIX: do not try to help CPython with garbage collection #23712

[rcomer]

I don’t know how much rearranging you generally do at this stage, but I wonder if this should go under “Figure and Axes creation / management”

[tacaswell]

• remove the errorbar markerfacecoloralt from api changes (it is in whats new)
• merge the contour/contourf new kwarg into the whats new entry (rather than an API behavior change)