Hide the language switcher unless it's relevant

[mattwynne]

Most of the pages in the current version of the docs don't vary according to your programming language, and it's misleading to show the language switcher tabs on those pages.

It would be better if we can have them only appear when we do have a page that has cross-platform docs on it.

@aslakhellesoy how would we go about doing that? We could disable the tabs by default and put a flag in the YAML frontmatter on the pages that do have cross-platform docs, for example.

[plaindocs]

If we use two templates, one with switcher and one without, we could manually turn it on for relevant pages

[aslakhellesoy]

Simply add polyglot: true to the front-matter to enable the language tab. It's off by default. It's now documented in CONTRIBUTING.md - see 2728a48