Move Changelog to standalone section; use the same pattern for Contributing

The changelog isn't a part of the main flow of introductory pages in the Documentation section, meaning that users aren't expected to read the changelog as part of getting started in the same way they are expected to read the other pages in the Documentation section. This patch moves the changelog to the end of the navigation structure to reflect this.

This patch also changes the structure of the Contributing page to be a standalone page instead of a directory, using the same pattern as the changelog.