feat(docs): rewrite theming guide, simplified docs styles (#DS-2304)

[NikGurev]

Summary

Turn theming.{en,ru}.md into a guide instead of a changelog: how the three
layers fit together, how to read theme values in your own styles, how to
override a component's variables and why :root cannot reach them, and how to
build a component on top of the theme. Document theme switching against
KbqThemeService — 'auto' follows the system color scheme out of the box, so the
manual matchMedia recipe is gone — and list only .kbq-light/.kbq-dark as
selectors. Add a small live example.

Inline _*-theme.scss partials into their components' own stylesheets so
the docs app stops demonstrating the pattern the guide argues against; only
library-level includes stay in _theme-kbq.scss. Where the wrapping .docs
selector had been carrying the weight, rules get a real structural selector
instead.

[github-actions]

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