Repair Doxygen build infrastructure

After Gerrit 4430 much of the documentation failed to build, but
silently so it was easily missed; equally missing that several
paths have been missing for a while.

- Correct paths after directory tree changes.
- Doxygen now bails when input paths don't exist.
- Fix up some of the less deranged entries in the documentation index.
- Exclude the LUA tree, its documentation is a mess.

Change-Id: I35e6b433feee5e05bca772d93aa1635c724db734
Signed-off-by: Chris Luke <chrisy@flirble.org>

README.md

@@ -30,22 +30,22 @@ Directory name         | Description
      build-root        | Build output directory
      doxygen           | Documentation generator configuration
      dpdk              | DPDK patches and build infrastructure
-     g2                | Event log visualization tool
-     perftool          | Performance tool
-@ref plugins           | VPP bundled plugins directory
-@ref svm               | Shared virtual memory allocation library
+@ref plugins           | Not-yet-relocated VPP bundled plugins directory
+@ref src               | VPP source code
+@ref src/plugins       | VPP bundled plugins directory
+@ref src/svm           | Shared virtual memory allocation library
+     src/tests         | Unit tests
+     src/vat           | VPP API test program
+@ref src/vlib          | VPP application library
+@ref src/vlibapi       | VPP API library
+@ref src/vlibmemory    | VPP Memory management
+@ref src/vlibsocket    | VPP Socket I/O
+@ref src/vnet          | VPP networking
+@ref src/vpp           | VPP application
+@ref src/vpp-api       | VPP application API bindings
+@ref src/vppinfra      | VPP core library
      test              | Unit tests
-@ref vlib              | VPP application library source
-@ref vlib-api          | VPP API library source
-@ref vnet              | VPP networking source
-@ref vpp               | VPP application source
-@ref vpp-api           | VPP application API source
-     vppapigen         | VPP API generator source
-     vpp-api-test      | VPP API test program source
-@ref vppinfra          | VPP core library source
-
-(If the page you are viewing is not generated by Doxygen then
-ignore any @@ref labels in the above table.)
+@ref vpp-api           | Not-yet-relocated API bindings
 
 
 ## Getting started