Add a portal to other subpages in the sidebar

[CloneWith]

This is to provide a more convenience interface for readers.
For example, when viewing Game modifier / Half Time (mod), it's better to make links to other related articles, which can be articles under the same parent directory, or have the same / similar tag, appear at the left side of the main content or on the top / bottom of it for better navigating.

[Walavouchey]

another approach would be to use infoboxes to accomplish the same thing, similar to how it's done on wikipedia

otherwise duplicate of ppy/osu-web#2764

[CloneWith]

I agree with this since we have lots of space on the left sidebar 😁

[CloneWith]

I thought it looks more likely to be an feature request for osu-web. However we also need preparation for this by writing simple articles for this.