refactor(markdown-magic): Extract transformations into their own modules

[Josmithr]

Also...

• simplifies pattern used for template-based transforms.
• adds missing README documentation for the new import instructions transform

[Josmithr]

No test changes, and no changes when running npm run build:readme from the root. No functional regressions: validated 😎

[tylerbutler]

This should have triggered a client CI build, but didn't. #21775 should fix that. If you manually validated that should be enough for this PR.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +245 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	455.4 KB	455.44 KB	⯅ +35 Bytes
azureClient.js	553.01 KB	553.06 KB	⯅ +49 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	256.72 KB	256.73 KB	⯅ +14 Bytes
fluidFramework.js	385.28 KB	385.29 KB	⯅ +14 Bytes
loader.js	134.01 KB	134.03 KB	⯅ +14 Bytes
map.js	42.17 KB	42.17 KB	⯅ +7 Bytes
matrix.js	145.43 KB	145.44 KB	⯅ +7 Bytes
odspClient.js	520.81 KB	520.85 KB	⯅ +49 Bytes
odspDriver.js	96.94 KB	96.96 KB	⯅ +21 Bytes
odspPrefetchSnapshot.js	42.23 KB	42.24 KB	⯅ +14 Bytes
sharedString.js	162.51 KB	162.52 KB	⯅ +7 Bytes
sharedTree.js	375.74 KB	375.75 KB	⯅ +7 Bytes
Total Size	3.24 MB	3.25 MB	⯅ +245 Bytes

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Baseline commit: 05193dc

Generated by 🚫 dangerJS against eee0b27