Add "Running Locust with Docker" to the TOC

[TBBle]

Without this, the only way to find the page is by either search, or happening on the link at the bottom of "Running Locust distributed".

Luckily, Google had indexed the page despite not being in the TOC, or I'd have never seen it.

[codecov]

Codecov Report

Merging #1221 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
+ Coverage   78.72%   78.87%   +0.15%     
==========================================
  Files          20       20              
  Lines        1955     1955              
  Branches      308      308              
==========================================
+ Hits         1539     1542       +3     
+ Misses        335      333       -2     
+ Partials       81       80       -1

Impacted Files	Coverage Δ
locust/runners.py	79.34% <0%> (+0.75%)	⬆️

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[heyman]

Thanks!