doc: Move existing rst files into api sub-directory

Currently the Sphinx doc only contains API descriptions of several
U-Boot subsystems. For future extension, group these existing docs
into an API sub-directory.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/efi.rst b/doc/api/efi.rst
similarity index 100%
rename from doc/efi.rst
rename to doc/api/efi.rst
diff --git a/doc/api/index.rst b/doc/api/index.rst
new file mode 100644
index 0000000..d484c06
--- /dev/null
+++ b/doc/api/index.rst
@@ -0,0 +1,11 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+U-Boot API documentation
+========================
+
+.. toctree::
+   :maxdepth: 2
+
+   efi
+   linker_lists
+   serial
diff --git a/doc/linker_lists.rst b/doc/api/linker_lists.rst
similarity index 100%
rename from doc/linker_lists.rst
rename to doc/api/linker_lists.rst
diff --git a/doc/serial.rst b/doc/api/serial.rst
similarity index 100%
rename from doc/serial.rst
rename to doc/api/serial.rst
diff --git a/doc/index.rst b/doc/index.rst
index 0353c10..1946d09 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -4,8 +4,16 @@
 U-Boot Developer Manual
 #######################
 
-.. toctree::
+U-Boot API documentation
+------------------------
 
-   efi
-   linker_lists
-   serial
+These books get into the details of how specific U-Boot subsystems work
+from the point of view of a U-Boot developer.  Much of the information here
+is taken directly from the U-Boot source, with supplemental material added
+as needed (or at least as we managed to add it - probably *not* all that is
+needed).
+
+.. toctree::
+   :maxdepth: 2
+
+   api/index