feat(v2): add support for RTL direction

[lex111]

Motivation

Do not merge this PR util facebookincubator/infima#61 is approved.

Adding support for the dir tag is mainly to allow RTL direction.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Not ready yet.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

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Size Change: +19 B (0%)

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Filename	Size	Change
website/build/assets/css/styles.********.css	17.6 kB	0 B
website/build/assets/js/main.********.js	109 kB	+15 B (0%)
website/build/blog/2017/12/14/introducing-docusaurus/index.html	21.7 kB	+4 B (0%)
website/build/docs/introduction/index.html	180 B	0 B
website/build/index.html	6.94 kB	0 B

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⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	74
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

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[slorber]

Oh that's cool, will try to test that soon! Thanks for working on this

We should rather add this direction to the i18n.localeConfigs because on localized sites you must be able to configure that direction on a per-locale basis:

https://v2.docusaurus.io/docs/next/docusaurus.config.js#i18n

[lex111]

Ah, yes, really makes sense, done.

[slorber]

Was able to remove the rtl theme overrides from datagit.ir site, add rtl config, and it seems to work

Will investigate a bit more on Monday, but it seems pretty good for now!

thanks

[lex111]

We also do not forget to make RTL styles for the built-in Docusaurus components. For achieve this, I suggest using the PostCSS plugin, which will be added via a new hook (see #4185).
In this PR I have already started applied new PostCSS hook, so after that PR is merged, deploy of current PR will be restored.

[slorber]

Thanks @lex111 that looks good to merge!

Will test it soon on datagit.ir in https://github.com/massoudmaboudi/datagit_v2.docusaurus/pull/2