Merge changes from PR then tag new release

[jlperla]

• [x] Merge #133 after review
• [x] tag a new version so the stable points to the help with the docs
• [x] make sure to rebuild the notebooks and pdfs for both the tagged version and dev (and test it works with stable
• [x] add link to stable derivation on front readme https://quantecon.github.io/SimpleDifferentialOperators.jl/stable/generated/discretized-differential-operator-derivation.pdf i.e. fix #136

[chiyahn]

Closing as the new version is now out: https://github.com/QuantEcon/SimpleDifferentialOperators.jl/releases/tag/v0.4.2