docs: clarify explicit grid tick values

[cat0825]

Summary

• Clarify that an explicit grid mark computes its own ticks when data is omitted.
• Explain that scale-level ticks are used by the implicit grid created via the scale grid option.
• Add an explicit Plot.gridX(ticks) / Plot.axisX(ticks) example for manually aligned ticks.

Fixes #2378

Verification

• pnpm exec prettier --check docs/marks/grid.md
• git diff --check
• pnpm run docs:build