User Guide: Html tags description

[alereds94]

in user guide many examples refer to html tags in strings as:
https://pytest-html.readthedocs.io/en/latest/user_guide.html#additional-summary-information

def pytest_html_results_summary(prefix, summary, postfix):
    prefix.extend(["<p>foo: bar</p>"])

by doing that, the <p> tag as well as other html tags (e.g. <div> or <h2>) are recognized as text.

The correct example would be to use py.xml package:

from py.xml import html

def pytest_html_results_summary(prefix, summary, postfix):
    prefix.extend([html.p("foo: bar")])

Another useful example would be the one to add a log to the Results Summary:

from py.xml import html

def pytest_html_results_summary(prefix, summary, postfix):
    postfix.extend([html.div("My additional log", class_='log')])

the User Guide shall be updated according to the correct html way to add information to the report sections.