change directory structure to match index

Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>

docs/developer_guide/hardware_reference/usb.md

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+In the **FunKey S** device, the USB interface has 2 functions:
+
+ - provide an external power supply source for both powering the
+   device and charging the built-in LiPo battery
+- provide a data interface to transfer firmware upgrades,
+  configuration files, game emulators and game ROMs
+
+The first function only requires the +5V USB power and GND pins. The
+second function requires to wire the additional differential data
+lines D+ and D-. As we only need to operate as an USB device and
+although the V3s is able to work as either an USB host or USB device
+using the USB OTG protocol, we don't need the ID pin to determine by
+the cable wiring which role we must take.
+
+![U02-BFD3111B0-009](/assets/images/U02-BFD3111B0-009.png){: align=left }
+
+The main part is of course the [Micro USB edge-mounted connector][1]
+**J2**, which we chose in order to "mask out" its already low-profile
+height into the PCB thickness. And with its "harpoon-like"
+through-hole legs, it should avoid tearing it off the board if the
+user don't pull the chord straight.
+
+The USB schematic is the following:
+
+![USB Schematics](/assets/images/USB_Schematics.png){.lightbox}
+
+Before connecting 2 devices using an USB cable, they may be at
+completely different absolute voltages, and during cable insertion,
+the shield will be in contact before the other pins, including
+GND. The **C6** capacitor between the Protective Earth (Shield) and
+GND is here to provide an AC path for sinking this difference in
+voltage and align the GND levels when plugin the cable.
+
+The resistor **R4** on the USB ID connector pin should probably not be
+mounted: as we act only as an USB device, this pin should be left
+floating.
+
+The capacitors **C7**, **C8**, **C10** and ferrite bead **L2** form a
+constant-k 3 pole CLC low pass filter to remove any spurious in/out on
+the USB power supply wire. The USB 2.0 specification limits the
+maximum bulk capacitance value to 10 µF in order to avoid power supply
+excessive droops when plugin in a device with a discharged large bulk
+capacitor.
+
+**D15** is a [NXP PRTR5V0U2X,215][2] combined TVS protection diode for
+the VBUS pin and a set of clamping diodes that will limit the voltage
+on D+ and D- pins to stay between GND and VBUS levels to in order to
+protect the V3s USB driver from under / over-voltages.
+
+[1]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/C40958_MICRO5P%E6%B2%89%E6%9D%BF%E5%BC%8F0.8%E5%9B%9B%E8%84%9A%E5%85%A8%E6%8F%92%E6%97%A0%E5%AF%BC%E4%BD%8D%E6%9C%89%E5%AD%94%E8%80%90%E9%AB%98%E6%B8%A9_2016-05-20.PDF
+[2]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/C12333_PRTR5V0U2X%2C215_2017-10-31.PDF
+
+--8<--
+includes/glossary.md
+--8<--