Add `getLaunchPadUrl()` utility function so Launchpad links never break again

Our 'Launch guide' link on the Sites page was broken for over a month because of https://github.com/Automattic/wp-calypso/pull/69561. Once we noticed the issue, it was fixed with https://github.com/Automattic/wp-calypso/pull/70885.

The underlying problem is that manually constructed URLs can be brittle. We should introduce a `getLaunchPadUrl()` utility function and use it throughout, so Launchpad pad links never break again.

**Done is:**
* `getLaunchpadUrl()` is introduced in some appropriate location.
* All links into the Launchpad use the utility function.

Start at the Sites page's broken “Launch guide” link, then search wp-calypso for other manually constructed links into Launchpad. Introduce getLaunchPadUrl() in an appropriate location and replace all Launchpad links with it. Done means the utility exists and all links into Launchpad use it.

Written by the indexing model from the issue text.