docs: document SCF namespace and API naming policy

[faisalahammad]

Adds docs/concepts/namespace-migration.md to record the naming policy for issue #150. The page keeps existing public ACF functions, hooks, classes, and constants stable, uses scf_ and scf/ for new code, and describes the staged migration criteria.

It also documents the compatibility boundaries that have to be checked before any rename: plugin slug, acf.php and secure-custom-fields.php, documented public APIs, internal implementation, and compatibility code. It covers the legacy mode tradeoffs from #92 and the requirement that security fixes stay in one common code path where possible.

The page is linked from docs/concepts/index.md and docs/contributing/index.md, and is registered in the regenerated docs/bin/manifest.json.

Closes #150

Testing instructions

• Open docs/concepts/namespace-migration.md and confirm the policy covers the public API, filename, and migration scope.
• Confirm docs/concepts/index.md and docs/contributing/index.md link to it.
• Confirm docs/bin/manifest.json contains concepts/namespace-migration.
• npm run lint:md
• composer test:php
• npm run test:unit

Docs only. No runtime behavior changes and no release zip changes. E2E and manual UI tests were not run because no plugin code was touched.

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