Updated a readme for react and this repository

[BEASTBOLTT]

Fixes #2301 — JSX (and JavaScript) code examples were not displaying correctly on the generated webpage despite rendering fine in the GitHub README viewer.

Problem

Fixes: #2301
All fenced code blocks across README.md and react/README.md used 4-space-indented fence markers:

• Some list item text...

  // bad
  return <div>{this.state.hello}</div>;

The CommonMark spec defines any line indented by 4 or more spaces as an indented code block. Many Markdown-to-HTML converters (including the one used to build the style guide webpage) interpret the 4-space-prefixed ```jsx lines as literal text inside an indented code block rather than as fenced code block delimiters. This caused:

The jsx / javascript markers to render as visible text on the page
JSX/HTML tags like

to bleed into the rendered HTML instead of being escaped inside a block
Syntax highlighting to break entirely
Fix
Moved all opening and closing fence markers to column 0 (no leading indentation), which is unambiguously parsed as a fenced code block by all CommonMark-compliant renderers:

• Some list item text...

    // bad
    return <div>{this.state.hello}</div>;

## Changes
| File | Change |
|---|---|
| `README.md` | De-indented 150× ` ```javascript`, 1× ` ```diff`, and 151 closing ` ``` ` fences |
| `react/README.md` | De-indented 25× ` ```jsx` and 25 closing ` ``` ` fences |
All 176 fence pairs remain **balanced** after the change (verified).
## Testing
- [x] No 4-space-indented fences remain in either file
- [x] All opening fences have a matching closing fence (balanced)
- [x] `README.md` and `react/README.md` render correctly in GitHub's Markdown preview
- [x] Markdownlint passes (`npm run lint`)
## References
- Fixes #2301
- CommonMark spec on indented code blocks: https://spec.commonmark.org/0.30/#indented-code-blocks
- CommonMark spec on fenced code blocks: https://spec.commonmark.org/0.30/#fenced-code-blocks