Improved site navigation

[NateWr]

I saw this header structure from Ubuntu's docs and I thought this could work really well for us once we implement some of the features we want for the docs hub, like better document structure, language-to-language navigation, search, etc.

Some notes on what I liked:

• The three-row header would work well for us to break the site down into sections. Top tier could be PKP | Documentation, next tier could be a nav menu depending on whether we're in the user or dev docs, and the last tier could be a breadcrumb for the current document.
• On the last tier, on the right, we could add a language switcher (and in the dev docs, a version switcher).
• The actual docs site (https://ubuntu.com/tutorials/command-line-for-beginners) breaks up long documents with hash tags (#5-moving-and-manipulating-files) which could be good for some of our long pages on Learning OJS 3.

[NateWr]

Two more examples suggested by Marisa. Adobe's use of "popular topics" on the homepage.

RedHat's use of filtering for discovery of documents.