docs: add E2E test tier documentation to contributing guide

[ArangoGutierrez]

Summary

• Documents the two-tier E2E test system (smoke pre-merge, full post-merge) introduced in PR ci: split E2E tests into smoke (pre-merge) and full (post-merge) tiers #740
• Adds a Label Taxonomy reference table covering all single-node and cluster labels
• Explains the post-merge label convention and provides guidance on which tier new tests should target
• Includes a "Running E2E Tests Locally" section with example make commands

Test plan

• Verify rendered Markdown tables display correctly in GitHub UI
• Confirm all labels listed match tests/aws_test.go and tests/aws_cluster_test.go
• Confirm smoke/full tier label filters match workflow YAML files

[github-actions]

Pull Request Test Coverage Report for Build 23739612933

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 56.346%

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Totals
Change from base Build 23589115472:	0.0%
Covered Lines:	3729
Relevant Lines:	6618

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💛 - Coveralls