docs(v2): trim, dedupe, and split the docs for clarity

[V3RON]

What is this?

A cleanup pass over the v2 docs (website/docs/v2) so pages only cover what a Voltra user actually needs to use the library correctly, without implementation internals, duplicated explanations, or overlong pages mixing unrelated topics.

How does it work?

• Cut "how it works under the hood" internals that don't affect usage: the internal wire/payload format, architecture diagrams, internal library/crypto names, and SwiftUI-modifier/Canvas-rendering explanations.
• Removed content duplicated across pages (plugin config blocks, "how build assets work" lists, repeated caveats like the iOS refresh throttle or Android's "opens the app when clicked") in favor of one canonical explanation with a link.
• Rewrote formal or marketing-toned openers into plain, direct language across the getting-started and platform intro pages.
• Split three overlong pages that mixed unrelated topics: ios/development/styling.md into styling/gradients/custom-fonts, android/api/plugin-configuration.md into config/widget-sizing-and-previews, and android/development/managing-ongoing-notifications.md (522 lines) into local-lifecycle vs. remote/push integration.
• Added expo-widgets to Prior Art as a practical (not just theoretical) example of driving Live Activities/widgets from a secondary JS runtime.
• Fixed the voltra CLI install snippet: --save-dev isn't supported by yarn or bun, only -D works across npm/yarn/pnpm/bun.
• Fixed a few bugs found along the way: a broken code fence rendering two sections as a literal code block, a page that existed but wasn't reachable via the sidebar, missing RN imports in a quick-start sample, and dangling links left by the page splits.

Why is this useful?

Shorter, non-duplicated, jargon-free pages are faster to scan and less likely to drift out of sync with each other. The page splits make each topic independently linkable and easier to maintain going forward.