Revising sorted collections documentation

[oxinabox]

Originally posted by @StephenVavasis in #479 (moved here since not directly related to the high level roadmap to 1.0)

I would like to make some small revisions to the documentation for sorted containers (see e.g. #528). I don't know how to do this, i.e., I don't know which file I should edit because I don't understand how the docs are organized and populated. Any pointers would be appreciated.

[oxinabox]

@StephenVavasis Sorted container docs are in https://github.com/JuliaCollections/DataStructures.jl/blob/master/docs/src/sorted_containers.md

The docs are built locally via:

1. Navigate to the docs directory .cd docs
2. Start julia with the docs directory set as the active project. julia --project=.
3. Run ] up (can skip this if already done recently)
4. Run: include("make.jl")
5. Files will appear in docs/build/index.html

CI runs something similar, but also deploys them

[StephenVavasis]

Closed via #787