Staging elasitc nav feature flag.

[reakaleek]

Context

The web team wants us to test the new elastic nav from their staging environment.

Request

Change the script in

docs-builder/src/Elastic.Documentation.Site/Layout/_Header.cshtml

Line 6 in 236e61e

<script src="https://www.elastic.co/elastic-nav.js" async></script>

to use https://stag-www.elastic.co/elastic-nav.js.

However, use a feature flag that is only enabled in our edge environment. See https://github.com/elastic/docs-builder/blob/main/config/assembler.yml#L30