Why my about page in Pelican returns 404?

Question

I cannot figure out Pelican. I want to add the /about page. I added the about.md file to /content/pages:

Template: about.html
title: about us
# this is content of about us

and I have a template: themes/simple/templates/about.html:

{% extends "base.html" %}
{% block content %}
    <h1>About Us</h1>
{% endblock %}

in my pelicanconf.py I added:

PAGE_URL = '/{slug}.html'
PAGE_SAVE_AS = '/{slug}.html'

when I go to localhost:8000/about I got 404. What am I doing wrong?