chore: Remove unused npm packages

## What

Remove npm packages from `src/Elastic.Documentation.Site/package.json` that have zero imports in the source tree.

## Packages to remove

| Package | Why |
|---------|-----|
| `uuid` | Zero imports; code already calls `crypto.randomUUID()` natively |
| `tippy.js` | Zero JS imports; only a leftover CSS reference remains |
| `moment` (devDep) | No imports anywhere |
| `text-diff` (devDep) | No imports anywhere |

## How

Remove each entry from `package.json`, run `npm install`, then `npm run build` to confirm nothing breaks. For `tippy.js`, audit the leftover CSS in `styles.css` and remove or inline the one or two rules needed.

Part of the broader ponytail over-engineering audit tracked in #3574.