* 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

Closes #766