Add comparison matrix to Velocity comparison page

[Malfrador]

A question that gets asked often is "Should I use Velocity, Waterfall or BungeeCord". While the existing page already answer that, its a lot to read and contains plenty of details that most server owners just looking for a simple answer aren't interested in.

This PR adds a small markdown table with a comparison matrix to the top of that doc page. It makes it easy to see at a glance what Velocity does, and what it doesn't do - mainly, supporting plugins made for BungeeCord, another question that gets asked very often.

Goal of this PR is to have an easy to understand overview for someone just looking for a simple answer to the aforementioned question. Those interested in more in-depth information can just continue reading the docs page.
With the table added, the docs page can simply be linked everytime someone asks that question, and contains a quick answer for them.

[olijeffers0n]

Two rows for velocity and bungee plugins seem redundant but i can’t think of a better way to do this. Also, surely there must be another pro to Bungee over velocity? Seems rather one sided that’s all 😄

[MiniDigger]

that table should somehow also show a difference between bungee and waterfall (not that I would know something to add there, lol)

also, maybe some kinda sentence introducing the table, bla bla, quick overview of the differences, read below for more detail.

[A248]

If you haven't any differences between BungeeCord and Waterfall, or you aren't willing to document them, you may want to remove one of the columns entirely.

There are differences. Waterfall guarantees Slf4j and exposes it in the API. On BungeeCord this library happens to be provided, but only due to the recent circumstance of library loading. Moreover, Waterfall has miscellaneous performance improvements to the implementation. I believe there might be a few new events. If we can get PaperMC/Waterfall#812 merged, then we can also say BungeeCord doesn't address some thread safety bugs -- which would be true.

By the way, Velocity can run some BungeeCord plugins via Snap. You may want to consider, thus, a mixed marker between a checkmark and an X.

Finally, in Velocity's favor, I strongly suggest adding a "Recommended by A248" row to your comparison matrix.