docs(vue): update useIonRouter replace direction - #4794

…nic-team#31583)

Issue number: resolves ionic-team#30617

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## What is the current behavior?

Currently, replacing a page inside a tab stack breaks going back in two
different ways. Calling `useIonRouter().replace()` shows a back button
that does nothing, because `replace()` sends the `root` direction, which
clears the location history but leaves `pushedByRoute` set. Calling
`navigate(path, 'forward', 'replace')` after switching tabs does the
opposite, so the URL goes back but the replaced page stays on screen.
That one happens because the back button calls `router.go(-3)` and the
router assumes the page we left is only one entry ahead.

## What is the new behavior?

Now `replace()` uses the `none` direction, so it keeps the pages beneath
the one it replaced and still doesn't animate, the same as a plain
`router.replace()`. The router also uses the real history delta to find
the page we left, and an explicit `navigate(url, 'root', 'replace')` no
longer shows a back button since there's nothing to go back to.

## Does this introduce a breaking change?

- [ ] Yes
- [X] No

## Other information

This does change `replace()` for apps that relied on it clearing the
history. That only started in 9.0, when ionic-team#31142 made the router respect
the caller's direction, and those apps can switch to `navigate(url,
'root', 'replace')`. I updated the `useIonRouter` docs in ionic-docs to
match here: ionic-team/ionic-docs#4794