Document SPDX license identifiers as recommended values for the manifest License field

### Description of the new feature/enhancement

Manifests currently provide almost no guidance on what to put in the `License` field of the `defaultLocale` manifest. As a result the field contains inconsistent free-text values (`MIT`, `MIT License`, `mit`, `Free`, `Proprietary`, `GPLv3`, etc.).

We should document that, when a package's license appears in the [SPDX License List](https://spdx.org/licenses/), contributors should use the exact SPDX **short identifier** (e.g., `MIT`, `Apache-2.0`, `GPL-3.0-or-later`) in the `License` field. For licenses that are not in the SPDX list, free text remains acceptable.

This came out of microsoft/winget-cli#3840.

### Proposed technical implementation details (optional)

- Update the manifest authoring documentation / contributing guide (and the manifest schema field description, if appropriate) to recommend SPDX short identifiers for the `License` field when one exists.
- Cross-link the [SPDX License List](https://spdx.org/licenses/).
- Pairs with a validation-pipeline enhancement that flags non-SPDX values and suggests the closest SPDX identifier (separate issue).

*Created with GitHub Copilot.*

Start with the manifest authoring documentation or contributing guide and the manifest schema description for the defaultLocale License field. Document use of exact SPDX short identifiers when available, link to the SPDX License List, and preserve free text for licenses not listed there.

Written by the indexing model from the issue text.