[chore] Upgrade to Docusaurus 3

[nmn]

What changed / motivation ?

Upgrade the documentation website to docusaurus 3. With some tweaks to the unplugin configuration, production builds are handled by the unplugin package while it still relies on runtime injection during development.

There was a small tweak needed to the unplugin package but there doesn't seem to be a regression in the webpack example as a result.

[github-actions]

workflow: benchmarks/perf

Comparison of performance test results, measured in operations per second. Larger is better.

[email protected] compare
node ./compare.js /tmp/tmp.FvZG14Iq9B /tmp/tmp.C6t9yFX1fL

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	558	542	0.97	-
· complex create	67	66	0.99	-
babel-plugin: stylex.createTheme
· basic themes	452	451	1.00	-
· complex themes	44	43	0.98	-

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.

[email protected] compare
node ./compare.js /tmp/tmp.ROZYOOHLmm /tmp/tmp.N0pIQm3fgn

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,311	1,311	1.00
· minified	4,150	4,150	1.00
@stylexjs/stylex/lib/cjs/inject.js
· compressed	1,223	1,223	1.00
· minified	3,216	3,216	1.00
benchmarks/size/.build/bundle.js
· compressed	496,650	496,650	1.00
· minified	4,847,840	4,847,840	1.00
benchmarks/size/.build/stylex.css
· compressed	99,853	99,853	1.00
· minified	747,541	747,541	1.00

[mellyeliu]

should we just wait for the rewrite a fumadocs? too many docs PRs