feat: add Spanish translation for Intro page

[huijing]

Part of #401

This is a replacement PR for #420 which takes the content from that PR and resolves the incorrect configuration issues.

To test this PR:

1. Go to https://deploy-preview-422--webmonetization-preview.netlify.app/docs/, there should be a language switcher dropdown now
2. Select Español and you should see the Spanish content rendered, with the tooltips also translated (the direct link to the page should be https://deploy-preview-422--webmonetization-preview.netlify.app/es/docs/)

[netlify]

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