* 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

These functions expect that file_path can have base_path stripped from
it, but during reloading they can be given relative paths.  Maybe this
behaviour varies between the notify backends?

This fixes two zola serve panics on FreeBSD (poll backend).

To ensure we will get the right `content`
directory.
Fix #629

Closes #275

Fix #215