Update application framework documentation

Update application framework documentation

Changes:
- Moved "Creating a New Service" page back over to "Developer Guides"
  since it is not application framework specific.
- Removed obsolete applaunchd D-Bus configuration documentation now
  that the functionality has been completely removed as of Pike.
- Updated the application framework introduction discussion around
  protocols to document preference for gRPC over D-Bus for AGL
  projects.
- Updated "Creating a New Application" documentation to document the
  requirements for having applications usable with applaunchd, as
  well as using the application templates, with BitBake recipe
  examples.
- Added "Application Sandboxing" section that documents the existing
  example configurations and discusses other potential configuration
  options for systemd.

Bug-AGL: SPEC-4928

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I6e0c0afcf571d9214df5300d2082b15fa3bb2a27
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/29284
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>