Add Feature Flags documentation

[ApexModder]

Initial rough write up of documentation for custom feature flags.

Goes over how to register and lookup a custom flag, mark elements as requiring said flag and how provide a feature pack to allow users to enable said flag.

Documentation can be found under NeoForge Documentation > Advanced Topics

This depends on either of the existing flag prs neoforged/NeoForge#1619 neoforged/NeoForge#1538

First time writing documentation like this, please let me know how/where I can improve anything.

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Preview URL: https://pr-184.neoforged-docs-previews.pages.dev

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[thomasglasser]

It may be better for readability to move the external section below the built-in section

[IchHabeHunger54]

Thank you for the contribution, and sorry for taking so long. This is looking quite good for a first version, the explanations are well done, and most change requests are of grammatical or structural nature.

A few things occurring across the entire documentation, which I'd ask you to change, are:

• The term "Feature Flags" is often capitalized. Terms like that should not be capitalized outside of headings. Otherwise we'd have to capitalize Block and Item every single time as well, which just looks weird.
• Please ensure that there is one blank line before and after each code block.
• Use H2s and H3s instead of H3s and bold paragraphs.