Development Documentation Refactoring

[kallisti5]

We really should reformat our developer documentation.

• User Guide
  • ​https://www.haiku-os.org/docs/userguide/en/contents.html
  • Generated from docs/userguide in git
  • Each language packed into haiku_userguide_XX hpkgs
  • "User Guide" Icon installed on desktop
• Developer Guide
  • ​https://www.haiku-os.org/docs/api/
  • Generated from docs/user in git
  • URL "api"
  • Called "Haiku Book" (aka like BeBook)
  • Not packaged, only on www.haiku-os.org.

Ideally, I think we should make the following changes:

• Move code to docs/developerguide
• Package like userguide (haiku_developerguide*.hpkg)
• Add an icon to the desktop "Developer Guide"
• Move document to haiku-os.org/docs/developerguide (and make /docs/api a redirect)

I honestly didn't even know where the developer guide lived given the odd / inconsistent naming within our sources.