Add schema.org microdata to the docs site

Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/145

• Meta description
• Schema.org Organization microdata
• Schema.org Website microdata
• Schema.org SearchAction microdata => https://gitlab.com/gitlab-com/gitlab-docs/issues/162
• Schema.org SiteNavigation microdata

Testing: View page source, select all, copy. Paste in https://search.google.com/structured-data/testing-tool. Or use the Review Apps URL http://add-schema-microdata.178.62.207.141.xip.io/

Below is the screenshot for how the search snippet displays before this implementation. We'll wait a few days to see how it looks after: