docs: Documenting built-in mixins

[rennerocha]

Continue work started on #133

[dariuschira]

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]

Codecov Report

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

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

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