docs: add warning about unmaintained example and sample code

[solnic]

This will silence dependabot alerts about gems and npm packages from sample rails apps that we have just for running tests and as examples in sentry-rails.

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.84%. Comparing base (213558f) to head (9cbfd09).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2646      +/-   ##
==========================================
+ Coverage   93.68%   96.84%   +3.15%     
==========================================
  Files         133      133              
  Lines        5101     5127      +26     
==========================================
+ Hits         4779     4965     +186     
+ Misses        322      162     -160     

Components	Coverage Δ
sentry-ruby	97.45% <ø> (+4.08%)	⬆️
sentry-rails	94.30% <ø> (+0.88%)	⬆️
sentry-sidekiq	94.67% <ø> (-0.77%)	⬇️
sentry-resque	94.44% <ø> (ø)
sentry-delayed_job	94.68% <ø> (ø)
sentry-opentelemetry	98.62% <ø> (-0.69%)	⬇️

see 33 files with indirect coverage changes

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