Clean up link checking

[blink1073]

Move link checking to a separate job and use new check-links action from jupyter_releaser.

[codecov-commenter]

Codecov Report

Merging #569 (9158dfe) into master (559ea43) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #569      +/-   ##
==========================================
+ Coverage   76.91%   76.95%   +0.04%     
==========================================
  Files         109      109              
  Lines        9951     9951              
  Branches     1079     1079              
==========================================
+ Hits         7654     7658       +4     
+ Misses       1914     1911       -3     
+ Partials      383      382       -1     

Impacted Files	Coverage Δ
jupyter_server/config_manager.py	94.52% <0.00%> (-2.74%)	⬇️
jupyter_server/serverapp.py	65.91% <0.00%> (-0.19%)	⬇️
jupyter_server/services/kernels/handlers.py	62.96% <0.00%> (+1.58%)	⬆️
jupyter_server/tests/services/kernels/test_api.py	99.01% <0.00%> (+1.96%)	⬆️

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