Wrap long lines

This commit wraps long lines in the Markdown source files. The main
motivation for this is to make it easier to view changes in Git commits.
Git shows line-by-line changes, so if a line contains an entire
paragraph, it'll be hard to see what has changed.

Most lines are now wrapped around the 80-character mark. This makes it
much easier to view changes in Git.

For some pages, this commit extracts link targets to the bottom of the
page in order to allow for more sensible line wrapping. The content of
pages is not changed.