chore: refactor tests

[tatomyr]

What/Why/How?

Refactored tests:

• moved benchmark -> tests/performance
• moved smoke tests under tests/smoke/
• moved e2e tests under tests/e2e/
• updated performance (https://github.com/Redocly/redocly-cli/pull/2390/files#diff-ebd18337ce8dc8b96a8403d2731d05bbb7502293037672a62faa622e36d2821a -> https://github.com/Redocly/redocly-cli/pull/2390/files#diff-9e1ce6c3421862cabaae4b2a65f12403f7bee2464c264a1abeed0afb001bf2ca), e2e, and unit tests setup
• removed run-smoke--yarn--node-20--redoc smoke test as Node v20 is no longer supported for Redoc CE
• updated the split unit tests to produce artefacts in the output/ folder so they are not visible
• CONTRIBUTING file updated accordingly

Triggered Technical Writers' review due to a README moving.

Reference

Previous attemt (abandoned due to a massive amount of conflicts): #2225

Testing

All tests work as before.

Documentation tests are failing due to big number of file changes.

Screenshots (optional)

Check yourself

• Code changed? - Tested with Redoc/Realm/Reunite (internal)
• All new/updated code is covered by tests
• New package installed? - Tested in different environments (browser/node)
• Documentation update considered

Security

• The security impact of the change has been considered
• Code follows company security practices and guidelines

[changeset-bot]

⚠️ No Changeset found

Latest commit: 20b8d93

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[github-actions]

Command	Mean [s]	Min [s]	Max [s]	Relative
redocly lint resources/rebilly.yaml	1.457 ± 0.028	1.417	1.512	1.00
redocly-next lint resources/rebilly.yaml	1.473 ± 0.029	1.435	1.545	1.01 ± 0.03

[DmitryAnansky]

Approved, please consider shorter subfolder names
tests/smoke/smoke-plugins -> tests/smoke/plugins