0.3.1: NitroText docs page, consistent benchmark figures, changelog heading

[ronickg]

The last changes before bun release for 0.3.1.

Docs

• NitroText has its own page in the sidebar. It was a section of the NitroInput reflow page, which now links to it.
• Getting started lists NitroText and NitroTime; it said "two components".
• The root README header names the morphing label.

Benchmarks

• A NitroText section on the docs Benchmarks page and in BENCHMARKS.md, including the method (the bench's touch-free text plan).
• All NitroText figures (docs page, both READMEs, changelog) now come from the same case on both phones: 1000 labels mounted into reused views that last showed other text. The earlier figures mixed that with reuse of the same text, which draws nothing.

Warm mount of 1000, main thread	iPhone 11 Pro	Galaxy A22
NitroText	113 ms	510 ms
react-native-plain-text	112 ms	620 ms
Text	140 ms	790 ms

Version

• CHANGELOG.md heading ## 0.3.1; the release notes script requires it.
• The example's "New in" labels say 0.3.1.

Not included

margelo/nitro#1660, a nitrogen fix for Android view prop updates, was tried in this library by patching nitrogen and regenerating. It measured no difference here, all within noise in 3 interleaved Release runs on the A22:

• NitroText mount: 510 vs 480 ms.
• Market screen, roll: 55.2% vs 53.7% main thread.

Our views' own work dominates each update, so it stays upstream only.

Checks

Docs build, typecheck, Jest 171/171. No native code changed.

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