Add links to the previous and subsequent lessons at the top and bottom of each lesson

[taylorreiter]

For an example, see the top of 08_bin_completion_with_spacegraphcats.md, which links the previous less.

In markdown, a link looks like this:

[previos lesson](https://github.com/dib-lab/dib_rotation/blob/master/07_taxonomic_discovery_with_sourmash.md)

where the word(s) that will be hyperlinked occur between the brackets, and the link occurs between the parentheses.