feat(macros/DefaultAPISidebar): add Tutorial section

[wbamberg]

Summary

This adds support for multi-page tutorials to DefaultAPISidebar, which will enable us to get rid of two custom sidebar macros: CanvasSidebar and WebGLSidebar.

Problem

Almost all pages in Web/API use one of two sidebars: DefaultAPISidebar for API overviews and guides, and APIRef for everything else. However there are four custom sidebars which exist only for a single API each.

Two of the custom sidebars are CanvasSidebar and WebGLSidebar. I would like these custom sidebars to be retired, and for us only to use the normal ones. However, these sidebars have one feature which we can't currently support in DefaultAPISidebar: a multi-page tutorial:

This is distinct from single-page guides and should not be dumped in there.

Solution

This PR adds a feature to DefaultAPISidebar that looks for a "tutorial" key in GroupData, that is expected to contain a multi-page tutorial given in reading order. If DefaultAPISidebar finds one of these it renders a "Tutorial" section in the sidebar.

This PR doesn't have a hard dependency on mdn/content: it can be merged without any corresponding mdn/content PR and should not break anything.

However, to see the "Tutorial" section it will need an mdn/content update. I've pushed an update to https://github.com/wbamberg/content/tree/use-defaultapisidebar-tutorials so you can see it working. The mdn/content update does three things:

• add a "Tutorial" entry to L10nCommon.json, so we can label the "Tutorial" section
• add a "tutorial" component to the GroupData for WebGL
• update the WebGL overview page to use DefaultAPISidebar instead of the custom sidebar

If this PR is accepted I will update all the WebGL and Canvas docs to use DefaultAPISidebar, then deprecate their custom sidebars.

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