Reduce documentation build time

[nathanshammah]

Issue Description

The documentation build time has roughly doubled from 30 minutes to about 60 minutes after adding the classical shadows documentation.

Proposed Solution

Some data could be copied in a file and not really generated at documentation build, especially for the section "1. How do I use classical shadows estimation?", which takes a long time to build.

[bdg221]

@Misty-W please assign this issue to me.

I have done some research and found:

I am first looking at the examples/rshadows_tutorial.md. I have taken the files that are pulled from the github repository and put them into a docs/source/resources folder. I tested locally and did see some speed up, though I was not seeing the same delays that the build from a PR sees. I'll be submitting a PR for testing purposes.

Also, I did leave a commented out version of the code using the repo.

[purva-thakre]

@bdg221 Some parts of guide/shadows and rshadows_tutorial rely on quantum processing of the circuits (number of snapshots) which takes a lot of time.

If the output of these quantum processing steps is saved as a resource (perhaps in the form of a text file?), it could help shorten the time because RTD does not have to rerun those codeblocks for every commit.

Pinging Misty-W natestemen andreamari just in case

Edit: Never mind, I did not see the linked PR which was already trying out something similar.