docs: describe how transforms reach the pod today

[catinspace-au]

DESIGN.md and MIGRATION.md said transforms are fetched from Artifactory by an init container. Nothing in the chart or the source does that, and an outside deployer has no such store. Docs only, no release.

How it works today:

• The chart has no init container, no download step and no transform volume.
• The wrapper loads transform YAML from config.transforms.dir (or the ordered config.transforms.files list). The image creates that directory empty at /etc/dfe-transform-vector/transforms.
• The chart mounts only config.yaml (subPath), so the directory stays visible. tests/integration/deployment.rs asserts this.

What changed:

• DESIGN.md: the volumes block of the architecture diagram shows the real config and transforms paths.
• DESIGN.md: the chart parity table no longer says the ConfigMap carries transforms/.
• MIGRATION.md: the "fetched from Artifactory" bullet now describes the transforms directory.
• MIGRATION.md: the "transform delivery stays the same" bullet is gone, and the changes table has a Transform delivery row.
• MIGRATION.md: the leftover mention of an artefact-store credential is removed.

Done when no doc names an init container or an artefact store for transforms.

[github-actions]

Released in v1.0.46 -- https://github.com/hyperi-io/dfe-transform-vector/releases/tag/v1.0.46