Eliminate flickering on page load

When navigating from page to page in the Docs, there is a flicker from the page loading the content. Ideally, the flickering should be eliminated.

From @robin13:
> Any page you open... e.g. https://www.elastic.co/guide/en/elasticsearch/reference/current/watcher-api-deactivate-watch.html

> I mean... you can do it in any browser, but here's a recording:
https://drive.google.com/file/d/14PVqnVb3lP8eq0gtEhpy-qRpZaW3cQH4/view?usp=sharing

Original ticket: https://github.com/elastic/website-www.elastic.co/issues/4293