docs: Add theming recipes

[nmn]

What changed / motivation ?

Adds three more recipes all related to theming.

1. Reset Themes

Explains how using stylex.createTheme(vars, {}) creates a theme that resets variables to their default values.

2. Merge Themes

Shows how to define object constants outside stylex.createTheme calls to be able to reuse and merge objects and avoid repetition.

3. light-dark

Talks about using the light-dark() function to control the color-scheme instead of defining three light, dark and system themes explicitly.

[github-actions]

workflow: benchmarks/size

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

@stylexjs/[email protected] size:compare
./size-compare.js /tmp/tmp.Wovxou5S9K /tmp/tmp.Mig4rKXsXX

Results	Base	Patch	Ratio
stylex/lib/stylex.js
· compressed	729	729	1.00
· minified	2,541	2,541	1.00
stylex/lib/StyleXSheet.js
· compressed	1,266	1,266	1.00
· minified	3,776	3,776	1.00
rollup-example/.build/bundle.js
· compressed	567,110	567,110	1.00
· minified	10,232,512	10,232,512	1.00
rollup-example/.build/stylex.css
· compressed	100,628	100,628	1.00
· minified	755,718	755,718	1.00