Correctly store language and platform mappings together (#3588)

Previously they were stored separately in their own arrays.
This made them disconnected, and impossible to know if a page
belonged to which platform and language combination.

This PR adds an array of objects called `targets` which contains
platform/language pairs.

.gitignore

@@ -17,3 +17,4 @@ test_result
 # but it's been superseded by a static asset
 # hosted at https://tldr-pages.github.io/assets/index.json
 pages/index.json
+index.json