[DOC] improved readme links

[fkiraly]

• reorganizes the top of the readme for a more orderly presentation of badges.
• migrates all badges to shields.io for consistency
• adds community links to sktime developer channels

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.14%. Comparing base (df3c386) to head (279e0f7).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1658   +/-   ##
=======================================
  Coverage   90.14%   90.14%           
=======================================
  Files          32       32           
  Lines        4780     4780           
=======================================
  Hits         4309     4309           
  Misses        471      471           

Flag	Coverage Δ
cpu	90.14% <ø> (ø)
pytest	90.14% <ø> (ø)

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