Refactor Call to Action directive to use chevron-right after dropping Pytorch

[Eric-Arellano]

For our call to action item, we need a right purple chevron:

I implemented that in ... by using a url() with an inline SVG, similar to how we do it in Pytorch.

But we already define a chevron in HTML:

qiskit_sphinx_theme/src/qiskit_sphinx_theme/theme/qiskit-sphinx-theme/partials/icons.html

Lines 20 to 28 in 86535e3

	<symbol id="svg-arrow-right" viewBox="0 0 24 24">
	<title>Expand</title>
	<svg version="1.1" id="icon" xmlns="http://www.w3.org/2000/svg" x="0px" y="0px"
	viewBox="0 0 32 32" style="enable-background:new 0 0 32 32;" xml:space="preserve">
	{#- We add `stroke` so that setting the color in outer CSS rules works, which Furo does. -#}
	<polygon points="22,16 12,26 10.6,24.6 19.2,16 10.6,7.4 12,6 " stroke="currentColor"/>
	<rect id="_x3C_Transparent_Rectangle_x3E_" fill="none" width="32" height="32"/>
	</svg>
	</symbol>

We should reuse this icon and use CSS to style it, e.g. change the stroke color.

I only didn't do this because it would be too complex to have a split implementation in Furo vs. Pytorch.

[Eric-Arellano]

This blocks the ecosystem because it blocks #487. We need to have the stroke color be var(--color-brand-primary).