docs: add make target for serving docs locally in docker

[shreddedbacon]

General Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for inclusion in changelog

Database Migrations

• If your PR contains a database migation, it MUST be the latest in date order alphabetically

Just adds make docs/serve to start docs locally without having mkdocs installed

[rocketeerbkw]

Would you also update the docs at https://docs.lagoon.sh/contributing-to-lagoon/documentation/?

[shreddedbacon]

Would you also update the docs at docs.lagoon.sh/contributing-to-lagoon/documentation?

Ah yep, will update the docs too

[rocketeerbkw]

Thanks!