docs: document new `ffa` directories

Document the `inc/ffa` and `src/ffa` directories in the "Project
Structure" docs.

Change-Id: I7b603a4c1b951851c80157e940b1b83ce806edfc
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/docs/getting_started/project-structure.rst b/docs/getting_started/project-structure.rst
index 52812e2..c1f5d9d 100644
--- a/docs/getting_started/project-structure.rst
+++ b/docs/getting_started/project-structure.rst
@@ -20,7 +20,8 @@
     ├── inc
     │   ├── hf
     │   │   ├── arch
-    │   │   └── plat
+    │   │   ├── plat
+    │   │   └── ffa
     │   ├── system
     │   └── vmapi
     │       └── hf
@@ -29,6 +30,7 @@
     ├── project
     ├── src
     │   ├── arch
+    │   └── ffa
     ├── test
     │   ├── arch
     │   ├── hftest
@@ -60,6 +62,8 @@
         separate implementations per platform. This includes details of the boot
         flow, and a UART driver for the debug log console.
 
+      - `ffa`: Interface for FF-A features.
+
    - `system`: which are included by the `stdatomic.h` which we use from
      Android Clang but not really needed, so we use dummy empty versions.
 
@@ -81,6 +85,8 @@
 
   - `arch`: Implementation of architecture-dependent modules.
 
+  - `ffa`: Abstraction over SPMC/hypervisor specific implementation details of FF-A features.
+
 - `test`: Integration tests
 
    - `arch`: Tests for components of Hafnium that need to be run on a real architecture.