feat(v2): blog + docs multi-instance plugins

[slorber]

Motivation

We should support multi-instance for the existing plugins, docs + blog.

Namespacing createData by pluginName/pluginId (it could work without, if file names are all hashed, but namespacing seems safer to me)

Dogfooding:

• add /second-blog as an example
• migrate community sidebar to a separate docs plugin (unversioned): /community/support

The docs versioning cli should work for both docs instances in a retrocompatible way.

I choose to add different command names, as we'd otherwise register twice the same command, and it would complicated to generate some kind of generic --pluginId option without any breaking change.

  docs:version <version>                                   Tag a new docs version
  docs:version:community <version>                         Tag a new docs version (community)

Other things done in this PR:

• refactor docs tests to make them more maintainable
• move some validation logic to docusaurus/utils-validation
• redirect old URLs like /docs/next/team

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Tests + Dogfooding:

Docs

Only the blog multi-instance has been documented a bit.

I don't document docs multi-instance now (or provide path customization options), because I want to do some breaking changes (particularly moving sidebars.js inside the docs folder, and renaming it to ./config.js)

[docusaurus-bot]

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