Add new Sidebar Container block to handle sticky sidebar logic

[ryelle]

This pulls out the sticky-related JS from the table of contents block, and ports over some CSS from the child themes, so that the sidebar pattern with table of contents can be reused — currently this component-pattern is used on Documentation, the new Developer site (in progress), and soon to be on the About subpages. By consolidating the code here, we can prevent duplicating the CSS across the child themes.

Additionally, this removes the use of aside for the sidebar, so it addresses some of the issue in WordPress/wporg-documentation-2022#58.

See WordPress/wporg-documentation-2022#63 for how this will be used in a child theme— we can remove the CSS and simplify the template. If this goes forward, I'll make a similar PR for Developer.

Before

<!-- wp:group {"tagName":"aside","className":"sidebar-container"} -->
<aside class="wp-block-group sidebar-container">
	<!-- wp:template-part {"slug":"search"} /-->

	<!-- wp:wporg/table-of-contents {"style":{"elements":{"link":{"color":{"text":"var:preset|color|blueberry-1"}}},"spacing":{"padding":{"top":"var:preset|spacing|20","right":"var:preset|spacing|20","bottom":"var:preset|spacing|20","left":"var:preset|spacing|20"}}},"backgroundColor":"blueberry-4","textColor":"blueberry-1"} /-->

	<!-- wp:paragraph {"fontSize":"small","className":"is-link-to-top"} -->
	<p class="has-small-font-size is-link-to-top"><a href="#wp--skip-link--target">↑ Back to top</a></p>
	<!-- /wp:paragraph -->
</aside>
<!-- /wp:group -->

After

<!-- wp:wporg/sidebar-container -->
	<!-- wp:template-part {"slug":"search","theme":"wporg-documentation-2022"} /-->

	<!-- wp:wporg/table-of-contents {"style":{"elements":{"link":{"color":{"text":"var:preset|color|blueberry-1"}}},"spacing":{"padding":{"top":"var:preset|spacing|20","right":"var:preset|spacing|20","bottom":"var:preset|spacing|20","left":"var:preset|spacing|20"}}},"backgroundColor":"blueberry-4","textColor":"blueberry-1"} /-->
<!-- /wp:wporg/sidebar-container -->

[StevenDufresne]

This is working as expected and the separating makes sense. 👍

I doubt this was introduced in this PR, but in safari there is an inconsistent behaviour where it doesn't properly attach: https://d.pr/i/tvomVC

I'll open a new ticket if you think it's unrelated.

[ryelle]

I doubt this was introduced in this PR, but in safari there is an inconsistent behaviour where it doesn't properly attach: https://d.pr/i/tvomVC

It works for me in Safari, both on production and with the new sidebar container. I tried toggling the reader view and reloading a few times, and it kept working. Can you open a new issue with more steps to reproduce?

In the meantime, I'm going to merge this so I can rebase & iterate on #389.