fix: links to Keyman engine content

[Meng-Heng]

Part of: #415.

There is no documentation from the $stable_version so I am adding a / to ensure the dropdown of other available versions.

Before:

After:

This PR is ready for review.

[darcywong00]

$stable_version is defined in PHP here

keyman.com/_includes/2020/KeymanVersion.php

Line 10 in f39a743

public const stable_version = '17.0';

I think the issue is since 17.0 is the stable version, the links are expecting https://help.keyman.com/developer/engine/iphone-and-ipad/17.0/ content.

Adding the slash will still be a broken link because iphone-and-ipad/16.0 is the "latest" folder that exists. I think we need to seed 17.0 and 18.0 folders in the help.keyman repo

[mcdurdin]

I think we need to seed 17.0 and 18.0 folders in the help.keyman repo

using current-version avoids this problem, and is the canonical link.

[darcywong00]

lgtm