Add documentation for new compression schemes

[huangsam]

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Description

Attempting to fill in the missing gaps for Celery user guides regarding newly supported compression schemes. I added spacing between the compression schemes so as to allow more room for scheme sub-sections if necessary. This should resolve #5328.

[huangsam]

This is how the documentation looks with the changes.

[huangsam]

Interestingly enough, running make html on the Sphinx docs with Python 3.7.2 fails while running make html with Python 3.6.6 works fine with a couple of warnings. Not sure if that's because of the particular Sphinx version that Celery is using under the hood.

[auvipy]

good start

[codecov]

Codecov Report

Merging #5384 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5384   +/-   ##
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  Coverage   83.39%   83.39%           
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  Files         144      144           
  Lines       16466    16466           
  Branches     2050     2050           
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  Hits        13731    13731           
  Misses       2526     2526           
  Partials      209      209

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[codecov]

Codecov Report

Merging #5384 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5384   +/-   ##
=======================================
  Coverage   83.39%   83.39%           
=======================================
  Files         144      144           
  Lines       16466    16466           
  Branches     2050     2050           
=======================================
  Hits        13731    13731           
  Misses       2526     2526           
  Partials      209      209

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 93afcb4...ac57fe0. Read the comment docs.

[huangsam]

The PR has been modified to include descriptions for each compression scheme - to help the user in making an informed choice on what is best for their particular use case(s).

[thedrow]

Looks good. The only thing that's missing is to say that Kombu has an extra dependency for most of those.
So pip install kombu[brotli] will install the appropriate brotli implementation.

[thedrow]

Done. Let's ensure that the build passes and merge this.