Reorganize the Developer's Handbook.

• Add chapter on kernel debugging (repo-copied from the Handbook)
• Change "Introduction" part with 4 empty chapters to a "Introduction" chapter with 4 mostly empty <sect1>s.
• Combine many of the different <part>s that contain only one empty <chapter> into larger <part>s.
• Add * in front of unfinished chapter names so its very clear what needs to be worked on.
• Added some <filename> tags where appropriate.