[repo] Add docusaurus cache

[andrewnicols]

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⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 79	🟢 95	🟢 100	🟢 90	🟢 100	Report
/docs/apis/commonfiles	🟠 72	🟢 93	🟢 100	🟢 100	🟢 100	Report
/general/development/gettingstarted	🟠 79	🟢 95	🟢 100	🟢 90	🟢 100	Report
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[kabalin]

Wondering, if we need to specify BUILD_FAST env var, which seems reduce test build to current version of docs only, Docusaurus is using it in GH workflows.

With regard to this change, if it does not have any aversive effects from .docusarus caching, LGTM.

[andrewnicols]

I'd like to avoid the BUILD_FAST as long as possible just because it means we can't review changes on backported docs.

[sarjona]

Thanks for adding this improvement, Andrew! And also for clarifying the question that Ruslan raised ;-)
The patch looks good and I'm happy to merge it :-)