Add script for local doctests

[AshlinHarris]

No description provided.

[AshlinHarris]

For building documentation locally, I comment out the call to deploydocs() from docs/make.jl. Of course, having 2 nearly identical Makefiles in the repository isn't ideal. Instead, this StackOverflow page gives some context on maintaining local versions of files:

git update-index --assume-unchanged docs/make.jl

will ignore local changes, but docs/make.jl is rewritten after a pull. Alternatively,

git update-index --skip-worktree docs/make.jl

does the same but persists after a pull. Neither command has any effect on other users; anyone who wants the same behaviour needs to run one of these commands locally before editing docs/make.jl.