docs: add keptn certificate manager infos

[RealAnna]

Fixes #657
with this pr we should also signal the breakingchange so plz do not merge without

BREAKING CHANGE: The dependency on cert-manager has been removed in favor of a custom implementation. With these changes, the operator will be waiting for a certificate to be ready before registering the controllers. The certificate is generated as a k8s secret in the lifecycle-toolkit namespace and then loaded into an empty dir volume. The Keptn certificate manager will make sure to renew it (every 12 hours) and will take care of its validity every time the controller manager deployment resource changes.

[codecov]

Codecov Report

Merging #652 (a8e80f5) into main (d33af19) will decrease coverage by 0.09%.
The diff coverage is n/a.

❗ Current head a8e80f5 differs from pull request most recent head e32eee8. Consider uploading reports for the commit e32eee8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #652      +/-   ##
==========================================
- Coverage   57.33%   57.24%   -0.09%     
==========================================
  Files          88       88              
  Lines        7001     7001              
==========================================
- Hits         4014     4008       -6     
- Misses       2821     2825       +4     
- Partials      166      168       +2     

Impacted Files	Coverage Δ
...ptnworkloadinstance/reconcile_prepostevaluation.go	81.81% <0.00%> (-9.10%)	⬇️
...lers/lifecycle/keptnworkloadinstance/controller.go	80.09% <0.00%> (-1.36%)	⬇️

Flag	Coverage Δ
keptn-cert-manager	?
keptn-lifecycle-operator	53.05% <ø> (ø)
klt-cert-manager	67.50% <ø> (?)
scheduler	21.17% <ø> (ø)

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
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No Coverage information
0.0% Duplication

[StackScribe]

Nicely done! I made a few word-smithing suggestions for your consideration. Nothing that is required.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication