docs: broken and malformed links in ROADMAP, CONTRIBUTING, and package docs

### Describe the issue

A mechanical link audit found 7 broken or malformed links in repo/package documentation:

- `ROADMAP.md` line 103 — `[change]((https://github.com/aws/aws-cdk/issues/32010))` double parentheses render the link broken
- `CONTRIBUTING.md` line 1422 — `[default in new CDK TS-based apps]([url](https://github.com/aws/aws-cdk/issues/7475))` nested-bracket link renders as literal text
- `CONTRIBUTING.md` line 1323 — links `./packages/@aws-cdk/cloud-assembly-schema/CONTRIBUTING.md`, which no longer exists in this repo (package moved to aws/aws-cdk-cli)
- `docs/NEW_CONSTRUCTS_GUIDE.md` line 38 — `[Construct Hub](constructs.dev)` bare-domain link resolves as a relative path and 404s
- `packages/aws-cdk-lib/assertions/MIGRATING.md` line 8 — `[Migration Script](migration-script)` missing the `#` anchor
- `packages/aws-cdk-lib/cloud-assembly-schema/README.md` lines 26/41 — `./schema/cloud-assembly.schema.json` only exists in the aws-cdk-cli repo

### Links

Verified each intended target's current location (filesystem or GitHub API).

Start with the seven entries listed in ROADMAP.md, CONTRIBUTING.md, docs/NEW_CONSTRUCTS_GUIDE.md, packages/aws-cdk-lib/assertions/MIGRATING.md, and packages/aws-cdk-lib/cloud-assembly-schema/README.md. Compare each link with its verified filesystem or GitHub target, then run a link or documentation check. Done means all listed links render correctly and point to existing current targets.

Written by the indexing model from the issue text.