docs(scraps): Add drag handle best practices (#122551)

`dragHandle.mdx` covered the props and the accessibility contract but
not the part people actually get wrong, which is what to do with the
delta the handle reports. Adds a Best practices section drawn from what
`SplitPanel` and `useColumnResize` already do rather than general
advice: clamp the delta yourself since the handle holds no size, pass
back the size you rendered, accumulate through the drag and commit once
at the end, and check the drag moved before committing.

Also corrects a sentence that was wrong. The docs said setting
`isSizedFirst` to `false` keeps the cursor and the keys pointing the
direction the divider can move. It only changes the cursor.
`useDragSeparator` never passes it to `useDragMove`, and the keyboard
branch is pure screen direction, which is why `SplitPanel` flips the
sign itself. Anyone who trusted it and built a trailing edge panel got
an inverted drag and an inverted keyboard.

Frontmatter gains WCAG 2.5.8, which the 24px pointer target already
satisfies and just was not listed.

Docs only, no source changes. Closes DE-1366