feat: Added an explanation on reactions

[Strixen]

Description

The guide has several pages that are currently WIP.

I took the liberty to flesh out the Reactions portion. Am a bit unsure how to target the guide however. Reactions and its usage is somewhat basic knowledge,
and very easy to glean usage from the api docs. Therefore i took the approach that the person reading would be quite entry level with python.
This means explaining things that might seem obvious and using a lot of comments in the code snippet.

It mainly covers the on_raw_reaction events since they are probably the most useful.
But included a MessageCommand example to demonstrate working with the Reaction class since its kind of the main focus. Truth be told there's not really that much to be said about Reactions, so added things i found useful when working with reactions on my projects. Also includes a brief primer on how Emojis work seeing as they're pretty integral to Reactions and knowing how they are supplied is important to filter events correctly.

Includes links to other parts of the guide and github to reduce repeating information that needs to be maintained and agree with each other.