* Add path to `TranslatedContent`
This makes it possible to retrieve the translated page through the `get_page` function.
* Use TranslatedContent::path field in test_site_i18n
Use it with the `get_page` function to get a reference to the page object.
* fix the issue of generating the search index for multiple language
* updat docs for generating the search index for multiple language
* fix failed tests
* add tests for the search index of multiple language
