docs: added docs for migrating encrypted secrets (#30132)

docs/usage/assets/images/app-settings/add-a-secret.png

docs/usage/assets/images/app-settings/add-repo-secret.png

docs/usage/assets/images/app-settings/app-credentials.png

...ge/assets/images/app-settings/encrypted-secrets-error.png

...usage/assets/images/app-settings/org-and-repo-secrets.png

.../usage/assets/images/app-settings/org-settings-button.png

...usage/assets/images/app-settings/repo-settings-button.png

...ge/assets/images/app-settings/stored-secret-encrypted.png

...ge/assets/images/app-settings/stored-secret-plaintext.png

docs/usage/getting-started/app-secrets.md

docs/usage/getting-started/migrating-secrets.md

docs/usage/getting-started/private-packages.md