Fix documentation coverage regression

This MR addresses the documentation coverage issue reported in flake-parts-website#2026.

Changes

1. Add description field to gitlab.ci option (nix/gitlab-ci.nix:64)

   • Fixes the documentation coverage regression
   • Ensures the option is properly documented

2. Enable flake validation in CI (nix/ci.nix)

   • Imports and enables the flake validation module
   • Runs nix flake show and nix flake lock validation
   • Prevents future documentation regressions

3. Add flake-parts-website docs test job (nix/ci.nix)

   • Tests building flake-parts-website with current gitlab-ci code
   • Uses --override-input gitlab-ci . to catch issues early
   • Runs in the test stage

4. Update stage names to standard conventions (nix/ci.nix)

   • Changed "devour" stage to "build"
   • Changed "cachix" stage to "publish"
   • Aligns with default stageOrder in the module

5. Update ChangeLog for version 0.10.2

Testing

The new CI jobs will validate that:

• The flake evaluates correctly (nix flake show)
• flake.lock is up to date
• flake-parts-website builds successfully with this version of gitlab-ci

Fixes: https://github.com/hercules-ci/flake.parts-website/issues/2026