So many people don't know what they are talking about. You do not need an official dock at all. There are 2 requirements for the switch to output over HDMI. It requires a PD (Power Delivery) profile of at least 39W, and requires and MHL protocol connection. The MHL protocol is used by mobile devices to provide more power that normal to the HDMI port so it can display its picture. It is used for stuff like cell phones, tablets, and portable Nintendo systems! Read more about it here
Essentially you will need 3 things for your USB-C dock:
It must support at LEAST 39W Power delivery It must support MHL It must be powered by a charger (And cable) which can support at least 39W Power delivery.
So if you have a USB-C hub that does not work with the switch, it is missing one of those things.
When shopping for a hub, hubs should list these specs. Unfortunately not all of them do. Heres an example of a hub that supports up to 100W PD, but not MHL. They have a note in the ad that it is not compatible with Nintendo switch, probably because its lacking MHL support.
Here is an example of a hub which does not list its PD Wattage capabilities (Which sucks), however they do advertise it as being switch compatible, so we know it supports at least 39W. I also personally own this hub and can testify that it indeed works.
Now for the power brick to power your hub. Again you do not need an official adapter. The Switch draws power over 2 different profiles. 7.5 watts (5 volts @ 1.5 amps) and 39 watts (15 volts @ 2.6 amps). As long as you buy an adapter that can provide this, or has a Power Delivery capability greater than 39W, you are ok.
In case you were unaware, Wattage = Volts * amps. So 15 volts * 2.6 amps = 39 watts :). Bonus: The switch also supports 5v * 1.5a = 7w. This means we can charge our nintendo switch with a 7w USB-C adapter also. It will charger slower, and not be powerful enough to display output to a TV, but it can charge. And most people have 7-10w usb c chargers already from their cell phones! Anyway, when buying the power brick, you need to make sure it can power 39W through 15v @ 2.6 amps.
I ended up buying THIS adapter, which is ultra slim and works fine. Youll notice it supports 45WPD, so Im covered as the PD spec will negotiate with the Switch to provide the correct Volts and current (15v @ 2.6amps). You only need to worry about the adapter specifically working at 15v @ 2.6amps if you're buying a cheaper adapter WITHOUT power delivery (The official power adapter for example, does not use PD. It is set to draw the exact Voltage and Amperage the switch needs). This also means the official adapter is actually worse than the linked 45W PD delivery adapter as you are locked into those 2 profiles with the official adapter. With this other adapter, you could use PD to negotiate the required voltage and amperage from the device its charging (As long as that device supports PD). So I could use it to charge lots of other things, like my cell phone, laptop, etc etc.
For anybody who talked about bricked switches, and risking damage from non official products, are just fear mongering. Sure you could buy a 3rd party power adapter that is fake and actually provides too much wattage and burns out your switch, but that's highly unlikely as long as you avoid ultra cheap crappy looking adapters and read a couple reviews for the product you're buying. You could of course always use a multi meter to test an adapters output before you plug it into your switch to ensure its outputting correctly.