docs: add onboarding guides with diagrams for QE engineers

[rnetser]

What this PR does / why we need it:

Adds four onboarding documentation guides for new QE engineers joining the team:

• ONBOARDING.md — Getting started from zero to running first test
• HOW_IT_WORKS.md — Visual guide to repo architecture with Mermaid diagrams
• REPO_MAP.md — Directory map, fixture dependency tree, key files reference
• WRITING_TESTS.md — Step-by-step test writing guide following project conventions

All docs use Mermaid flowcharts that render natively on GitHub.

Which issue(s) this PR fixes:

Special notes for reviewer:

Draft — reviewing for accuracy and completeness. Known items to fix:

• ONBOARDING.md references non-existent `test_vm_lifecycle.py` path
• HOW_IT_WORKS.md incorrectly places `VirtualMachineForTests` in `libs/vm/vm.py`
• HOW_IT_WORKS.md test domains diagram missing `data_protection/`
• REPO_MAP.md hardcoded line counts will go stale
• REPO_MAP.md references `.flake8` but project also uses ruff

jira-ticket:

Assisted-by: Claude noreply@anthropic.com

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (9b6c72f) to head (fad0791).
⚠️ Report is 45 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4902   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          25       25           
  Lines        2485     2487    +2     
=======================================
+ Hits         2452     2454    +2     
  Misses         33       33           

Flag	Coverage Δ
utilities	98.67% <100.00%> (+<0.01%)	⬆️

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[openshift-virtualization-qe-bot]

D/S test tox -e verify-tc-requirement-polarion failed: cnv-tests-tox-executor/28090

[openshift-virtualization-qe-bot]

D/S test tox -e verify-tc-requirement-polarion failed: cnv-tests-tox-executor/28091

[rnetser]

/retest tox

[rnetser]

/retest tox

[rnetser]

/retest tox