"Installation" link updated/fixed

[miderose]

Prior link redirected to a 404 page.

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[nknapp]

Hi and thanks for the PR. Could you change the URL again, I think the "installation/index.html" page is more appropriate.

[nknapp]

I think it should link to "https://handlebarsjs.com/installation/".

[ole-martin]

I agree, the /guide/installation is just the quick-fix method for having something up and running, proper installation docs are indeed located at /installation/

[nknapp]

merged #1620 instead