fix language dropdown for getting-started pages

[bishabosha]

singlepage-overview layout has to dynamically lookup the English page from translations to find the list of other languages, but it is hardcoded to look inside the _overviews collection, so is broken when used as a layout on any translated page where the English page is not in the _overviews collection.

I did not find a way to systematically relate e.g. _es/getting-started/index.md to _getting-started/index.md, when we also have to relate _zh-cn/tutorials/scala-for-java-programmers.md to _overviews/tutorials/scala-for-java-programmers.md so the simplest thing is to move the source files for _getting-started to overviews/getting-started.

before:

after:

alternatives:

• making a new layout for getting-started - but why when we are not changing the styling, just more fragile things people must remember
• adding more metadata to front matter of getting-started translations to say which collection should be searched - not scalable and prone to people forgetting

[julienrf]

Great idea!