Entity docs

[KonSola5]

The long-awaited moment - Entity documentation is done.
Split off from #88, and only contains the method documentation part - I don't feel confident in writing texts explaining the basics, and those basics are what should be highlighted in "concepts" folder.

• Documented all methods from Entity that I deemed useful for script authors, this includes methods from base Entity class and KubeJS-injected interfaces,
• Documented SlotAccess and KubeRayTraceResult needed to document some Entity methods.

Notes

• Used CSS injection to add a custom "Optional" tag - KJS Wiki doesn't have a nice alternative for this.
• All references to Java classes are in placeholders in en.yml, so that if/when a class is documented, the static text can be easily swapped over for a link to article.
• I intentionally did not use beans in examples that use boolean-returning isX methods - from practice, usage of isX beans is not guaranteed to produce a boolean because of the fact that if a subclass decides to add a getX method into their class, the bean will refer to the return value of getX, and not isX, which can cause confusing bugs.