[rss] Display RSS Feeds in Notifications

[ricoberger]

It is now possible to use the RSS plugin to display the items from an
RSS feed within the notifications. RSS feed items are always marked as
unread and info.

We also added a page implementation for the plugin to show the items of
a single RSS feed. This allows us to link from a notification entry to
the complete rss feed.

• 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 #392 (9afed34) into main (184289e) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #392      +/-   ##
==========================================
+ Coverage   45.20%   45.21%   +0.01%     
==========================================
  Files         134      134              
  Lines       10377    10379       +2     
==========================================
+ Hits         4691     4693       +2     
  Misses       5608     5608              
  Partials       78       78              

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

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