fix(v2): throw error if first level item of a sidebar is not category

[endiliey]

Motivation

Close #1993

See that issue for more context

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

• Newly added test passes

[yangshun]

How about we allow non-categories for first-level items? Seems like a valid thing to do.

[docusaurus-bot]

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[docusaurus-bot]

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Built with commit 0fa178f

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[endiliey]

Hmm i don't know, maybe in the future. But honestly at this point i won't work on it for now until i finish versioning. That will require some quite refactoring on docs plugin (which will definitely conflict with my versioning PR #1983 and fixing conflict on my own is kinda sad. we'll have to update our documentation as well https://v2.docusaurus.io/docs/sidebar/#sidebar-object

This is only possible in v2 though, because v1 use object keys for the category :|
I am pretty sure most people are doing

// sidebars.js
module.exports = {
  docs: {
    'Getting started': ['greeting'],
    Docusaurus: ['doc1'],
  },
};

Especially for v1 users

[endiliey]

I'm gonna merge it first since that seems more like a feature request. If someone wanted to work on that, send a PR (don't forget tests). PS: it will need quite some refactor on order.ts, sidebars.ts and index.ts on docs plugin.!All typings must be updated as well since assumption is different now