Add rss plugin

[ricoberger]

• I added a CHANGELOG entry for this change.
• I adjusted the corresponding documentation for this change.
• I adjusted the values.yaml file and the documentation for all values.

[codecov-commenter]

Codecov Report

Merging #330 (dde6386) into rework-kobs-architecture (8ccf65b) will increase coverage by 1.36%.
The diff coverage is 100.00%.

@@                     Coverage Diff                      @@
##           rework-kobs-architecture     #330      +/-   ##
============================================================
+ Coverage                     49.17%   50.53%   +1.36%     
============================================================
  Files                            69       72       +3     
  Lines                          4539     4664     +125     
============================================================
+ Hits                           2232     2357     +125     
  Misses                         2251     2251              
  Partials                         56       56              

Impacted Files	Coverage Δ
packages/plugin-opsgenie/cmd/opsgenie.go	100.00% <ø> (ø)
packages/plugin-rss/cmd/rss.go	100.00% <100.00%> (ø)
packages/plugin-rss/pkg/feed/feed.go	100.00% <100.00%> (ø)
packages/plugin-rss/pkg/instance/instance.go	100.00% <100.00%> (ø)

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