Improved Deque Documentation + docstrings

[lfenzo]

Hello!

This is the first of a series of PRs aiming to dramatically improve the current documentation of DataStructures.jl.

Some of the changes include the addition of actual documentation generated by Documenter.jl from docstrings (see docs/src/deque.md), a brief explanation of the Deque data structure and a few minor changes in source code.

The new documentation page formats will be structured in the following manner:

1. Data structure description and characteristics.
2. Constructors for the implemented data structure
3. Usage, showing first a list of available methods for the given data structure sorted alphabetically, followed by the Documenter.jl generated documentation.

Let me know in case of any suggestions.

[StephenVavasis]

Glad to hear you are doing this! Please note that #787 completely rewrites the documentation for the sorted containers. So if you are planning to work on those docs, it would be better to wait for 787 to get merged.

[lfenzo]

Hello @StephenVavasis

Thanks for letting me know! My plan is to rewrite the whole documentation (or as close to that as possible) automating everything with Documenter.jl, so I'll reach that one at some point.

Btw, nice work in #787.

Anyone available for reviewing this?

[oxinabox]

fantastic, thanks