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: NBin Meng <bmeng.cn@gmail.com>
Reviewed-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

doc/api/index.rst

+.. SPDX-License-Identifier: GPL-2.0+
+
+U-Boot API documentation
+========================
+
+.. toctree::
+   :maxdepth: 2
+
+   efi
+   linker_lists
+   serial

doc/index.rst

@@ -4,8 +4,16 @@
 U-Boot Developer Manual
 #######################
 
+U-Boot API documentation
+------------------------
+
+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
 
-   efi
-   linker_lists
-   serial
+   api/index