Add breadcrumbs markup to our doc pages

[debadair]

From https://github.com/elastic/website-www.elastic.co/issues/83:

Adding this will allow things like Elasticsearch 1.4 > Aggregations > Boolean to be displayed in search results. Instructions can be found here:
https://developers.google.com/search/docs/guides/enhance-site#enable-breadcrumbs

There is renewed interest in doing this to uplift our Google search results.

@AnneB-SEO can provide more flavor from the SEO perspective and broader structured data efforts.

[gtback]

Another reference: https://developers.google.com/search/docs/data-types/breadcrumb