move contrib into buildbot-contrib repository

[tardyp]

updated the docs to move contrib links to the new repository

[mention-bot]

@tardyp, thanks for your PR! By analyzing the history of the files in this pull request, we identified @djmitche, @warner and @rodrigc to be potential reviewers.

[codecov]

Codecov Report

Merging #2827 into master will decrease coverage by -0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2827      +/-   ##
==========================================
- Coverage   87.44%   87.44%   -0.01%     
==========================================
  Files         312      312              
  Lines       32921    32921              
==========================================
- Hits        28789    28787       -2     
- Misses       4132     4134       +2

Impacted Files	Coverage Δ
master/buildbot/reporters/mail.py	94.87% <0%> (-0.57%)	❌

---

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 fe22ca0...2cd85b9. Read the comment docs.