Misc. documentation fixes

Some (mostly minor) documentation fixes (typos, formatting errors, outdated instructions) that I noticed while reading through the live 1.8.0 docs.

The most important fix is in CONTRIBUTING.md, where git commands weren't being rendered correctly. The fix was to add newlines between the numbered list entries (which also aids readability of the markdown).

The script usage testing docs were referencing output files, which no longer exist. Not sure how serious it is that this makes it into the live docs (probably not a huge deal).