added table and some MD formatting to 01-backup.md, deleted 05-undo.md

For this change to 01-backup.md, I have added the table that compares the `mode` option for the `git reset` command.  I also modified some of the Markdown syntax to match earlier parts of this document.  And finally, I removed the 05-undo.md document since its content is mostly presented in the 01-backup.md.