Restructure & improve documentation

[rytilahti]

• Add information about contributing
• Move device-specific documentation under their own section

[codecov-commenter]

Codecov Report

Merging #1139 (91a2c99) into master (981368c) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1139      +/-   ##
==========================================
- Coverage   74.87%   74.81%   -0.06%     
==========================================
  Files          76       76              
  Lines        8999     9006       +7     
  Branches      767      769       +2     
==========================================
  Hits         6738     6738              
- Misses       2077     2084       +7     
  Partials      184      184              

Impacted Files	Coverage Δ
miio/gateway/devices/subdevice.py	36.87% <0.00%> (-1.93%)	⬇️

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