ref(onboarding): Split unity onboarding doc into multiple files

[priscilawebdev]

Contributes to https://linear.app/getsentry/issue/TET-864/introduce-folders-for-onboarding-platforms

[linear]

TET-864 Introduce folders for onboarding platforms

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
12264	1	12263	10

View the top 1 failed test(s) by shortest run time

getPlatformPath legacy platforms (old structure) handles special cases

Stack Traces | 0.032s run time

Error: expect(received).toBe(expected) // Object.is equality

Expected: "language/language"
Received: "unity"
    at Object.toBe (.../utils/gettingStartedDocs/getPlatformPath.spec.ts:97:68)
    at Promise.finally.completed (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/jestAdapterInit.js:1559:28)
    at new Promise (<anonymous>)
    at callAsyncCircusFn (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/jestAdapterInit.js:1499:10)
    at _callCircusTest (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/jestAdapterInit.js:1009:40)
    at processTicksAndRejections (node:internal/process/task_queues:105:5)
    at _runTest (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/jestAdapterInit.js:949:3)
    at _runTestsForDescribeBlock (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/jestAdapterInit.js:839:13)
    at _runTestsForDescribeBlock (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/jestAdapterInit.js:829:11)
    at _runTestsForDescribeBlock (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/jestAdapterInit.js:829:11)
    at run (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/jestAdapterInit.js:757:3)
    at runAndTransformResultsToJestFormat (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/jestAdapterInit.js:1920:21)
    at jestAdapter (.../sentry/node_modules/.pnpm/jest-circus@30.0.4_babel-plugin-macros@3.1..../jest-circus/build/runner.js:101:19)
    at runTestInternal (.../sentry/node_modules/.pnpm/jest-runner@30.0..../jest-runner/build/testWorker.js:272:16)
    at runTest (.../sentry/node_modules/.pnpm/jest-runner@30.0..../jest-runner/build/testWorker.js:340:7)
    at Object.worker (.../sentry/node_modules/.pnpm/jest-runner@30.0..../jest-runner/build/testWorker.js:494:12)

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