Fix bug unable to search docs according to its version

[endiliey]

Motivation

As per https://docusaurus.io/docs/en/search#extra-search-options

Any occurrences of "VERSION" or "LANGUAGE" will be replaced by the version or language of the current page, respectively.

However, the current implementation is currently broken, while algolia docsearch expect something like '1.3.0', we provide 'current' as the version if we are currently in the latest stable version.

https://github.com/facebook/Docusaurus/blob/c5661b0e1e131803709852adc2283fb4894aabcc/lib/core/Site.js#L36-L43

https://github.com/facebook/Docusaurus/blob/c5661b0e1e131803709852adc2283fb4894aabcc/lib/core/Site.js#L130-L134

Docusaurus.io docsearch is working fine because we don't search according to current version.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Use this algolia index for testing and use version facetfilters

algolia: {
    apiKey: 'c53337307f1787a9199348cd8263f666',
    indexName: 'docusaurus_meta',
    algoliaOptions: {
      facetFilters: [ "language:LANGUAGE", "version:VERSION" ] 
    }
  },

Before (search is broken)

After this PR (search according to current version)

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[yangshun]

LGTM

[endiliey]

It's quite a serious bug actually. It's just that not many user know that they can refine their algolia search 😢 because it's quite complicated (you have to learn algolia config 😂 )

Btw I'll merge this & sleep. I'll fix the other PR tmrw @yangshun

[JoelMarcey]

@endiliey - With all the great fixes you and @yangshun have been putting out over the last couple of days, I think a 1.3.1 version is appropriate soon.