Handbook: next/prev links do not follow the sidebar order

[oandregal]

Outstanding differences:

• Data module has Packages as its next link and not its next subsection. Fixed by Consolidate doc generation and fix next/prev links issue #15421
• Packages has Components as its next link and not its next subsection.
• Components has Theming for Gutenberg as the next link and not its next subsection.
• The Outreach section (last of the handbook) has @wordpress/a11y package as the next section.

The prev/next links are created based on the ordering of sections/subsections within the manifest.json file. This file is something we create on demand.

[oandregal]

This is because the package, component, and data subsections are generated on the fly, and added to the manifest after this is created. We should instead:

• replace the existing placeholders in toc.json (see {{packages}}, {{components}}, and {{data}}).
• generate the manifest.json

[oandregal]

#15421 modifies how handbook data docs are generated and fixes the links for that section.

[oandregal]

There is #15456 to fix the remaining issue (documents and packages).