render docs for devices.qubit functions

[timmysilv]

I am open to notable variation in the docs here, just lmk! I mostly just wanted to get this PR going.

Context:
The devices.qubit functions are very useful, and people should be able to see them! They're still a little unstable, so I'm inclined to leave those warnings in the docs for now, but they should be visible.

Description of the Change:
Add the devices.qubit qubit-simulation functions at the bottom of the qml.devices sub-page in the "Internal" section (check it out here!)

Benefits:
Nice pretty docs to show users all the cool features and helper functions we have

Possible Drawbacks:
It's still kinda unstable while we ramp up DQ2, but it should be ready by the time we release 0.33

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2e3c671) 99.66% compared to head (cf5d91c) 99.66%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4555   +/-   ##
=======================================
  Coverage   99.66%   99.66%           
=======================================
  Files         376      376           
  Lines       33071    33071           
=======================================
  Hits        32961    32961           
  Misses        110      110           

Files Changed	Coverage Δ
pennylane/devices/__init__.py	100.00% <ø> (ø)
pennylane/devices/qubit/__init__.py	100.00% <ø> (ø)
pennylane/devices/qubit/adjoint_jacobian.py	100.00% <ø> (ø)
pennylane/devices/qubit/apply_operation.py	100.00% <ø> (ø)
pennylane/devices/qubit/preprocess.py	100.00% <ø> (ø)
pennylane/devices/qubit/sampling.py	100.00% <ø> (ø)
pennylane/devices/qubit/simulate.py	100.00% <ø> (ø)

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[albi3ro]

Could we add adjoint_jacobian, adjoint_jvp, and adjoint_vjp to the table of contents as well?