Add initial docs for Entity Pathfinders and Mob Goal API

[olijeffers0n]

I have not extensively used either of these API's in the past, however these seem to work

closes #265

[MiniDigger]

this needs notes on how vanilla selects which goals to activate and prolly some more caveats on how pathfinding works from somebody who actually uses the pathfinder, iirc it can ge all kinda wonky to call moveTo every tick.

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[Phoenix616]

This feels confusing as it kinda reads as if one could create Vanilla goals entities (especially under the register section) which isn't possible right now.

[zlataovce]

the casing is a bit wonky in several places beyond my comments, but otherwise lgtm

[zlataovce]

Suggested change

	## Creating a Custom Goal
	## Creating a custom Goal

[zlataovce]

Suggested change

	## Pathfinding Rules
	## Pathfinding rules

[Phoenix616]

As far as I'm aware of that's just the grammatical rules of titles in English. Basically everything but "a", "the", etc. should be uppercase if I'm not mistaken?

See Title case (but Wikipedia itself doesn't use that but sentence casing, which imo. Is easier to read but can feel unnatural in headings)

[zlataovce]

As far as I'm aware of that's just the grammatical rules of titles in English. Basically everything but "a", "the", etc. should be uppercase if I'm not mistaken?

See Title case (but Wikipedia itself doesn't use that but sentence casing, which imo. Is easier to read but can feel unnatural in headings)

we use sentence case in most places throughout the site