[docs][fonts][expo-font][icons]: Remove AppLoading references and update Snack examples

[amandeepmittal]

Why

Refs ENG-5822 & ENG-5874

How

This PR updates all examples using <AppLoading /> component in Using Custom Fonts, expo-font docs, Creating a splash screen, and Icons guide.

It also:

• replaces inline snack examples with embedded snacks
• restructures Using Custom Fonts guide to follow a user journey from starting point and leading to examples with further customization or features
• Changes the tile from Fonts to Using Custom Fonts since this has been a point of confusion
• Other examples that were previously using Class Components are updated to use function components (except where it is necessary to explicitly use a class component example)
• Uses <Terminal /> component for installation instructions instead of plain shell code snippets

Test Plan

Checklist

Changes have been tested by running the docs locally.

• [x] Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• [x] Conforms with the Documentation Writing Style Guide
• [ ] This diff will work correctly for expo prebuild & EAS Build (eg: updated a module plugin).

[linear[bot]]

ENG-5822 Remove AppLoading component references

The https://docs.expo.dev/guides/using-custom-fonts/ page references AppLoading component which is deprecated. It leads to a point of confusion.

Other pages that refer or recommend using AppLoading component (for asset caching)

• https://docs.expo.dev/guides/splash-screens/#using--apploading--andor--splashscreen
• https://docs.expo.dev/guides/icons/