Add headers at Markdown blank example

[rgaiacs]

@gvwilson I don't remember why our sections use h3. Do you remember?

[gvwilson]

When we stitch things together to make a book, h1 is the book title,
h2 is the chapter titles (e.g. "Python", "Git"), so h3 is the title
for the sections (e.g., the heading of novice/python/03-loop should be
an h3), and headings inside a lesson should all be h4.

[wking]

On Thu, Sep 18, 2014 at 03:49:42PM -0700, Greg Wilson wrote:

When we stitch things together to make a book, h1 is the book title,
h2 is the chapter titles (e.g. "Python", "Git"), so h3 is the title
for the sections (e.g., the heading of novice/python/03-loop should be
an h3), and headings inside a lesson should all be h4.

For what it's worth, HTML lets you just use

everywhere (thanks to

) [1](https://html.spec.whatwg.org/multipage/semantics.html#headings-and-sections).

[rgaiacs]

For what it's worth, HTML lets you just use

everywhere (thanks to

) [1].

I don't know if it will play nice with pandoc when we try to create the EPUB and
PDF version of the lessons.

And please don't merge it yet. I want to add the same text to the IPython
Notebook example.

[rgaiacs]

I notice that IPython Notebook template already has a note about this:

Use a Heading-3 for Subsection Headings

@gvwilson If this looks OK we could merge it.

@wking about <h1> and <section> I want to postpone this change so I could have time to play and find out how pandoc deal with it.

[gvwilson]

Keep this or discard it in the run up to #759?

[rgaiacs]

@gvwilson I'm merging this before #759.