Remove Page.content_type when we move PEPs to RtD

[berkerpeksag]

I noticed this while working on #1102. Page.content_type was added in #520. However, we could use Page.content.markup_type for the same purpose.

>>> rss = Page.objects.get(path='dev/peps/peps.rss')
>>> rss.content_type
'application/rss+xml'
>>> rss.content.markup_type
'restructuredtext'

• While doing this, we should also add a comment saying PageView.content_type is a public attribute of TemplateResponseMixin.content_type which is a base class of DetailView.
• And add a note that says templates/pages/raw.html is used to create /dev/peps/peps.rss.

[hugovk]

Will be fixed by #2552.