Documentation website build fixes

[taraspos]

Summary

Bunch of update to improve documentation website deployment:

1. Configure submodules as shallow for quicker build
2. Expose env variables DOCUSAURUS_CONFIG_URL and DOCUSAURUS_CONFIG_BASE_URL to allow dynamic configuration
3. Configure routeBasePath to / as per https://docusaurus.io/docs/docs-introduction#docs-only-mode
4. Update yarn build command to expose CLI params, to be able to use --out-build as per:
   • Documentation-only sites do not work if baseUrl is not / facebook/docusaurus#2404 (comment)
5. Remove hardcoded docs prefix from redirect urls

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