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/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