Add global language selector and docs page navigation filtering to improve docs DX

[chrisraygill]

Adds a global language selector to the docs navigation and filters the docs navigation column content in CSS based on selected language so that only relevant pages are displayed - reducing liklihood of developers finding irrelevant docs pages for a given language.

Global and page-level language selectors are in sync and both affect the navigation filtering.

Change should be SEO-friendly because all pages in the navigation are available to crawlers in the HTML, with filtering happening through CSS.