Add navigation banner to "classic" docs

[dedemorton]

Problem description

The serverless docs have a navigation banner that allows users to navigate back to the main docs landing page easily. The banner also has links to API and release docs:

The stateful docs do not have this banner. To navigate back to the main docs landing page, you need to click Elastic Docs in the breadcrumb trail.

Navigation is complicated by a couple of factors:

• The product-level landing pages (such as https://www.elastic.co/guide/en/observability/current/index.html) do not have the breadcrumb trail because those pages live outside of the generated pages for the "book."
• It's easy to accidentally trigger the Elastic site navigation when you try to click a link in the breadcrumb trail. This is super annoying because the popups are huge.

How to fix the problem

The fix for this problem is a quick win. We need to:

• Add the navigation banner to the HTML template used by doc build.
• Add the navigation banner to the product-level landing pages.
• Add the required styles to the docs CSS.
• For consistency, rename the breadcrumb text "Elastic Docs" to "Documentation" (or remove it)

Benefits of fixing this problem:

• Navigation across all doc pages is more consistent.
• Navigation back to the main docs landing page is easier.
• Links to the API and release docs appear on every page in the docs.
• Breadcrumb trail appears further down on the page and does not interfere with the site navigation.

[dedemorton]

For consistency, rename the breadcrumb text "Elastic Docs" to "Documentation" (or remove it)

Correction, we should use the home icon. While we have the file open, we might also want to change the breadcrumb separator from > go /.