doc: add partition API to HTML documentation

* Convert comments in part.h to Sphinx style.
* Create documentation page for the partition API.
* Add the partition API page to the API index page.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/api/index.rst b/doc/api/index.rst
index a9338cf..3a80ae0 100644
--- a/doc/api/index.rst
+++ b/doc/api/index.rst
@@ -15,6 +15,7 @@
    lmb
    logging
    nvmem
+   part
    pinctrl
    rng
    sandbox