[1.14.x] Rewrite docs for Registries

[sciwhiz12]

Rewrote [Concepts > Registries]:

• Removed references to GameRegistry.register
• Reordered register methods so DeferredRegister is first and foremost
• Simplified section on creating registries (if someone will make their own registry, they'll be capable of researching on their own)
• Changed ObjectHolder rules from block of text to rules list (TIL that final only matters if the class and not the field has @ObjectHolder)

Issue #275 can be closed, and I think the related issue #85 can be closed, since the issue is from long, long ago (2017), and both points are resolved:

1. GameRegistry.register in preInit is non-existent at this point; and
2. RegistryBuilder is only used when a modder wants to adds a registry, which is not common I would assume.

[Cyborgmas]

first review, and it seems that i do not have the hang of it. Got most stuff twice in each... sorry for the hassle

[Aeltumn]

Everything looks pretty good, I've only got two small comments. Although I've never used the @ ObjectHolder so I can't give any feedback on that part.

[sciwhiz12]

As 1.14.x is moving into out-of-support, I am closing this PR. See the updated PR for 1.15.x.

[sciwhiz12]

As said to me, 1.14.x is the branch to currently target. Since 1.14.x is essentially the same to 1.15.x in regards to registries, the reviews from the other branch will apply to this branch.

[sciwhiz12]

I might have to make a PR once this is merged into 1.15.x, to change the section on DeferredRegister from using the constructor to #create.