"gitlab" flake input type not documented

[aidalgol]

Problem

nix flake manual page does not mention gitlab as a possible input type, even though this is supported.

Checklist

• checked latest Nix manual (source) (could not locate file in source tree)
• checked open documentation issues and pull requests for possible duplicates

Proposal

Add an item under the "Types" section detailing the support for gitlab as a type, similar to the gitlab and sourcehut entries.

Priorities

Add 👍 to issues you find important.