Surface Sub-component Readmes in Handbook Documentation

[mkaz]

Numerous sub-components have readme and documentation created, but these are not surfaced in the Gutenberg Handbook, and thus can not easily be linked to.

One example is InnerBlocks, the component documentation is here:
https://github.com/WordPress/gutenberg/tree/master/packages/block-editor/src/components/inner-blocks

But the Gutenberg Handbook documentation for Block Editor shows as it is undocumented here: https://wordpress.org/gutenberg/handbook/designers-developers/developers/packages/packages-block-editor/

Which also means we can't link to it from places like here, which tries but is a broken link
https://wordpress.org/gutenberg/handbook/designers-developers/developers/block-api/block-templates/

[aduth]

Considering action steps here:

• Unless I'm mistaken, the process of including the document should be as simple as adding to manifest.json and re-running npm run docs:build
• There is already a "Components" top-level section in the handbook. Currently, these map directly to UI components offered by the @wordpress/components module. Would it be fine to include components from other modules (block-editor) as well, or does that risk confusion? Alternatively, could this be listed under the @wordpress/block-editor heading, or do we risk confusion on (a) having a very deep hierarchy and (b) fragmentation of components documentation.
• I'm assuming there's some "magic" behind the scenes to remap a link from /packages/block-editor/src/components/inner-blocks/README.md to an equivalent location in the handbook, and that this would automatically take effect to fix the "Block Templates" documentation once reachable.

[bph]

Now that the Developer Resources redesign has been put into production, this would be the next step to provide a complete and comprehensive documentation for developers.