Adding Schema.org link to sidebar

[KilimAnnejaro]

Per discussion with @tjann, I'm trying to migrate the content from https://github.com/datacommonsorg/data/tree/master/docs into links in the main docs repo. Most of that migration is in #63; this PR incorporates the Schema.org link from that page into the side navbar.
This PR is fundamentally a bit of a stopgap measure for two reasons:

1. I think that the entire structure of having a redirect page for this functionality is a bit weird. However, adding a link to the sidebar is non-trivial at this time--we will probably need a navigation.yaml to make that easier.
2. I dislike the lack of annotation for leaving Data Commons, and would like to model the link styling along the lines of https://www.isomer.gov.sg/ and its source https://github.com/isomerpages/isomerpages-isomergovsg.
   If you think it would be better to wait until I can restructure navigation to incorporate this link, feel free to close this PR; however, if it would be beneficial to incorporate the schema.org link into the link structure quickly (along with the rest of the content from https://github.com/datacommonsorg/data/tree/master/docs), then this PR will be useful.

[tjann]

Hi @KilimAnnejaro could you update this PR with the master branch, I'll take a look then. Thank you!

[KilimAnnejaro]

@tjann done!

[tjann]

Nice. Thanks!