Add documentation for supported functionality

[eddddddy]

Context:
In general, the amount of configurations for QNodes is quite large, and due to the supported 5 interfaces, it is quite hard to estimate a priori which scenarios are supported, which can not be supported for mathematical/programmatical reasons and which are simply not yet supported because they haven't been implemented.

Description of the Change:
Provides an overview in the documentation that informs users about the supported scenarios, and explains which scenarios are conceptually unsupportable and why.

Benefits:
Better documentation and faster lookups.

Possible Drawbacks:

Related GitHub Issues:
Closes #1997

[codecov]

Codecov Report

Merging #2540 (70090ba) into master (dd28293) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2540   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files         243      243           
  Lines       19595    19595           
=======================================
+ Hits        19513    19514    +1     
+ Misses         82       81    -1     

Impacted Files	Coverage Δ
pennylane/devices/default_qubit_torch.py	93.13% <0.00%> (+0.98%)	⬆️

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[antalszava]

Hi @eddddddy, this is a huge effort and will bring us forward a lot 💪 🎉

Not only does it provide info to users, but we also have a more systematic check-in place for cases that can go a long way for our testing practices.

Left some suggestions. No real blockers, rather polishing. One general thought is that if a functionality doesn't work as we'd like and there's an issue open (or will be opened), then linking the issue number/link in a comment can also help with a TODO marker. Also, in the issue itself, marking the parts of this addition can help because once the issue is resolved, those parts would also have to be updated.

[antalszava]

[sc-19547]

[antalszava]

Looks great @eddddddy! 🏆 Once the comments are addressed, this is ready to be merged. The ones marked as minor/as an idea are up for consideration, no definite need to change the behaviour there.