Add navigation links within individual notebooks about intro_programming

[ehmatthes]

Only do this on notebooks long enough that links are helpful.

[ehmatthes]

Reopening, because with longer notebooks such as Lists and Tuples that include lots of details I am not creating links for every section as I am writing. Easy pickings later on, though.

[ehmatthes]

This will also allow more nesting of links between notebooks. For example, I use the newline character "\n" in the section about looping through lists. I'm not confident new programmers will remember that, I can link directly to the part of Variables Strings Numbers that discusses whitespace.

[ehmatthes]

I'm hoping to solicit some collaboration shortly, so I want to use internal links as a guide for collaboration:

• If a link in the Contents section of a notebook is active, then that section has been drafted. Feel free to revise any drafted sections.
• If a link is inactive, that section has not yet been drafted. These are sections that are ripe for contributions.
• Notebooks with no Contents section are completely empty, and are open for contributions.