docs: Documenting built-in mixins

[rennerocha]

Continue work started on https://github.com/scrapinghub/spidermon/pull/133

[mirceachira]

Hey @rennerocha I've looking a bit into #133 and this and was wondering if the issue could be up for grabs?

I think documenting SpiderMonitorMixin should include some docs for spidermon.contrib.stats.counters.DictPercentCounter which is mixed with ResponseInfo from mixin class.

I'd be happy to contribute some docs to this.

[codecov[bot]]

Codecov Report

Merging #232 into master will not change coverage. The diff coverage is n/a.

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##           master     #232   +/-   ##
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  Coverage   69.28%   69.28%           
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  Files          67       67           
  Lines        2745     2745           
  Branches      306      306           
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  Hits         1902     1902           
  Misses        784      784           
  Partials       59       59

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