- Adds ability to build multiple projects simultaneously using, for example, `make docs PROJECTS='grafana grafana-cloud'`.
- Adds `make doc-validator` which runs [`doc-validator`](https://github.com/grafana/technical-documentation/tree/main/tools/cmd/doc-validator) on all documentation.
Using a centralized script will help ensure consistency in workflow across all projects.
Signed-off-by: Jack Baldry <jack.baldry@grafana.com>
