[DOCS] Added SDIO overview.

Author: jameswalmsley

_includes/docs_contents.html

@@ -54,6 +54,10 @@ <h4>API</h4>
         <a href="{{ site.url }}/docs/api/rtc">RTC Subsystem</a>
       </li>
 
+	  <li class="{% if page.title == "SDIO Subsystem" %}current{% endif %}">
+        <a href="{{ site.url }}/docs/api/sdio">SDIO Subsystem</a>
+      </li>
+
 	  <li class="{% if page.title == "Semaphores & Mutex's" %}current{% endif %}">
         <a href="{{ site.url }}/docs/api/semaphore">Semaphores & Mutex's</a>
       </li>

docs/sdio/sdio.md

@@ -0,0 +1,21 @@
+---
+layout: docs
+title: SDIO Subsystem
+next_section: usage-rules
+permalink: /docs/api/sdio/
+header_path: drivers/mmc/core.h
+source_path: drivers/mmc/sdcard.c
+---
+
+Manages SD Host controller interface devices, e.g. SDHCI or SPI based SD-card drivers.
+
+
+# Further Reading
+
+The full API is defined under:
+
+{% include api/header-path.md %}
+
+See the implementation under:
+
+{% include api/source-path.md %}