Add class hierarchy images to storage section

Add class hierarchy diagrams to storage APIs as test.

Author: AnotherButler

docs/reference/api/storage/ChainingBlockDevice.md

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 ## ChainingBlockDevice
 
+<span class="images">![](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/class_chaining_block_device.png)<span>ChainingBlockDevice class hierarchy</span></span>
+
 The ChainingBlockDevice class provides a way to chain together multiple block devices. You can then interact with the chained block devices as if they were a single block device of size equal to the sum of each substorage unit. The ChainingBlockDevice acts as an opposite of the [SlicingBlockDevice](/docs/development/reference/slicingblockdevice.html).
 
 Note that each block device's block size must be a multiple of the other devices' block sizes (512, 1024 and so on).

docs/reference/api/storage/HeapBlockDevice.md

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 ## HeapBlockDevice
 
+<span class="images">![](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/class_heap_block_device.png)<span>BlockDevice class hierarchy</span></span>
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 The HeapBlockDevice class provides a way to simulate block devices for software development or testing. The created blocks are nonvolatile; they do not persist across power cycles.
 
 HeapBlockDevices have the following configurable parameters in either one of two constructors:

docs/reference/api/storage/MBRBlockDevice.md

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 ## MBRBlockDevice
 
+<span class="images">![](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/class_m_b_r_block_device.png)<span>MBRBlockDevice class hierarchy</span></span>
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 The MBRBlockDevice class provides a way to manage a Master Boot Record (MBR) on a storage device, which allows you to partition the device. Without the MBR, you can still format a storage device with a file system, but including the MBR will allow for future partition modifications.
 
 MBRBlockDevices have the following configurable parameters in the constructor: