docs: set up Migration Guide

[StackScribe]

Part of #1499

This is a bare start to the project, just to set up the files to make content development go more smoothly. Comments are welcome but it probably is not worth your time at this point.

Note that this PR originally included more content and @agardnerIT provided some excellent comments. The content and Adam's comments have been preserved and will show up in subsequent PRs.

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[codecov]

Codecov Report

Merging #1506 (52a19f3) into main (72f5b82) will increase coverage by 20.00%.
The diff coverage is 84.30%.

❗ Current head 52a19f3 differs from pull request most recent head 1f08490. Consider uploading reports for the commit 1f08490 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1506       +/-   ##
===========================================
+ Coverage   63.37%   83.38%   +20.00%     
===========================================
  Files         142      131       -11     
  Lines       10822     8081     -2741     
===========================================
- Hits         6858     6738      -120     
+ Misses       3708     1094     -2614     
+ Partials      256      249        -7     

Impacted Files	Coverage Δ
metrics-operator/api/v1alpha3/keptnmetric_types.go	100.00% <ø> (ø)
...is/lifecycle/v1alpha3/keptntaskdefinition_types.go	100.00% <ø> (ø)
...ator/controllers/lifecycle/keptntask/controller.go	56.33% <66.66%> (ø)
...controllers/lifecycle/keptntask/runtime_builder.go	70.44% <70.44%> (ø)
...rator/controllers/lifecycle/keptntask/job_utils.go	73.94% <80.76%> (-0.22%)	⬇️
...ifecycle/keptntaskdefinition/reconcile_function.go	75.55% <81.25%> (ø)
...ollers/lifecycle/keptntaskdefinition/controller.go	80.43% <81.81%> (ø)
.../lifecycle/v1alpha3/keptntaskdefinition_webhook.go	93.44% <93.44%> (ø)
...trics-operator/api/v1alpha3/keptnmetric_webhook.go	90.47% <100.00%> (ø)
...erator/controllers/common/taskdefinitionhandler.go	100.00% <100.00%> (ø)
... and 3 more

... and 14 files with indirect coverage changes

Flag	Coverage Δ
certificate-operator	68.55% <ø> (ø)
component-tests	59.29% <54.09%> (-0.21%)	⬇️
keptn-lifecycle-operator	?
lifecycle-operator	84.48% <82.38%> (ø)
metrics-operator	82.31% <100.00%> (ø)
scheduler	30.45% <ø> (ø)

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
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