Add instruction about how to configure local version of docs

[temporallyaccount]

Add instruction about how to configure each local version of docs. It must include about home page, docs and blog pages and gridsome even able to offer local versions as well. I believe volunteers may interest to write docs in local languages while gridsome is very useful.
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[mtlynch]

Thanks for this suggestion!

The official issue for i18n is here:

gridsome/gridsome#23

But the docs website could do a better job of summarizing the current i18n options with Gridsome.