docs: update 10-9-custom-reports.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added the new PERCENT(num;denom) formula to the "Writing formulas" function table and tips, plus a note (here and in "Running a report") that percentage cells render with a % suffix and aren't clickable/drillable, unlike other cell types.

Source PRs

• axolotl#4517 — feat(GREEN-1413): render PERCENT report columns with a % suffix
• light#7351 — feat(GREEN-1413): native PERCENT(num;denom) report column formula

How to use this PR

1. Review the diff against the existing article — focus on accuracy and customer voice.
2. Merge if it reads well. Edit and push to this branch if it needs adjustment.
3. Close without merging if the article doesn't actually need this update.

Once we have a track record of clean drafts, we'll graduate to auto-merge with a delay buffer. For now: human-in-the-loop.