Fix problem with local search when site_url is not at the root

When the site url is not at the root, the localsearch plugin __init__.py needed to be modified slightly to include the folder when using the post permalink. In addition, I added some corresponding comments to the conf.py.sample.
If this produces an URL like `http://example.com/nikolasite/posts/welcome-to-nikola.html`, then you should do:

```py
                    data["loc"] = post.permalink(lang=lang, absolute=True)
```