Streaming docs

[DanSimon]

Fixes #553

First attempt at more thorough docs for the streaming API. This focuses more on the fundamentals of pipes, sources, and sinks, since I think once you understand those using the StreamingHttp protocol is more straightforward. I think there is still more to add but we can improve it incrementally. I'd also be interested to get people's opinions on what else should be covered.

I tried adding some links to the public scaladocs hosted by javadoc.io. Unfortunately it doesn't work with paradox's built-in scaladoc tag, but it seems to work nicely using an extref tag.

Writing up these docs uncovered some areas in the API that could be cleaned up and improved a bit, so I'll have another PR soon that'll address some of it.

[codecov-io]

Codecov Report

Merging #591 into develop will decrease coverage by 0.04%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop    #591      +/-   ##
==========================================
- Coverage    84.94%   84.9%   -0.05%     
==========================================
  Files           99      99              
  Lines         4431    4431              
  Branches       378     358      -20     
==========================================
- Hits          3764    3762       -2     
- Misses         667     669       +2

Impacted Files	Coverage Δ
...s/src/main/scala/colossus/metrics/Collection.scala	97.43% <0%> (-2.57%)	:arrow_down:
.../scala/colossus/metrics/collectors/Histogram.scala	90.57% <0%> (-0.73%)	:arrow_down:

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