docs: improve React-related sections in troubleshooting and plugins

[palpavasiya]

Description

This PR adds practical React-specific guidance to help developers better understand and avoid common pitfalls when using Vite with React.

Changes:

1. docs/guide/troubleshooting.md

   • Added a new subsection ### React Fast Refresh caveats under the HMR section.
   • Covers the most common reasons React components cause full reloads instead of hot updates:
     • Mixing component and non-component exports
     • Anonymous default exports
     • Class components

2. docs/plugins/index.md

   • Improved descriptions for @vitejs/plugin-react and @vitejs/plugin-react-swc.
   • Added a clear "Choosing between plugin-react and plugin-react-swc" tip box to help developers make the right choice.

These updates address frequent questions and pain points from the React community when using Vite.

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Why this is valuable:

• Reduces debugging time for thousands of React + Vite users
• Improves documentation clarity without changing any behavior
• Pure documentation improvements

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Notes:

• No breaking changes
• No code changes, only documentation