Update Alerts doc

[jotak]

Description

• Remove some details of the default alerts (thresholds & variants) , just describe the templates
• Add a section that describes metadata & the health annotation
• Move the embedded sample YAML into an actual sample

[openshift-ci]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.73%. Comparing base (dbce689) to head (4d583a6).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2070      +/-   ##
==========================================
+ Coverage   71.72%   71.73%   +0.01%     
==========================================
  Files          80       80              
  Lines       10677    10689      +12     
==========================================
+ Hits         7658     7668      +10     
- Misses       2617     2619       +2     
  Partials      402      402              

Flag	Coverage Δ
unittests	71.73% <ø> (+0.01%)	⬆️

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see 6 files with indirect coverage changes

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