docs(theme): document the slot-class replacer (function form) + "Theme component" section

[IgorShevchik]

Context

Follow-up to PR #183 (port of upstream nuxt/ui ea576e2f — feat(theme): allow replacing slot classes with a function, nuxt/ui#6562).

The runtime feature is fully ported and shipped in #183: a slot class can be a function (defaults) => classes that replaces the slot's default classes (instead of merging) via :b24ui, the class prop, or app.config.b24ui. Types (SlotClass / SlotClassReplacer), the tv wrapper rewrite, and tests are all in place and green.

During that port, two documentation-only pieces from upstream were intentionally skipped because b24ui's docs/skills are structured differently from nuxt/ui and a 1:1 insertion would not have mapped cleanly:

1. docs/content/docs/1.getting-started/5.theme/3.components.md — upstream added, in its global-config section:

   • a "By default these classes are merged… set a slot to a function to replace them entirely" explanation with framework-only (Nuxt app.config / Vue vite.config) code blocks;
   • a "Theme component" subsection describing scoped overrides via <B24Theme> (which does exist in b24ui as src/runtime/components/Theme.vue) and referencing the theme-ui-example (b24ui has docs/app/components/content/examples/theme/ThemeUiExample.vue), noting that slots accept the same function form to replace classes.

   In feat(theme): allow replacing slot classes with a function #183 only a focused note + :b24ui example + theme.unstyled tip was added to the existing ### b24ui prop section.

2. Skill guidance — upstream updated skills/nuxt-ui/references/guidelines/design-system.md with a "Replace instead of merge" section. b24ui's skill lives at skills/b24-ui-nuxt/ with different structure and was not updated.

Scope of this issue

• Expand 5.theme/3.components.md to document the replace-instead-of-merge function form at the global-config level (Nuxt app.config.b24ui + Vue vite.config bitrix24UIPluginVite), mirroring upstream but in b24ui's doc structure.
• Add/align a "Theme component" (<B24Theme>) section noting that slot overrides accept the (defaults) => classes replacer form (the theme-ui-example already exists).
• Update the b24ui skill under skills/b24-ui-nuxt/ with the equivalent "Replace instead of merge" guidance.

References

• PR: feat(theme): allow replacing slot classes with a function #183
• Upstream commit: ea576e2f321665574c416af34e7ef04e68654a79 (feat(theme): allow replacing slot classes with a function nuxt/ui#6562)
• Existing assets in b24ui: src/runtime/components/Theme.vue, docs/app/components/content/examples/theme/ThemeUiExample.vue

Target

Target start: 2026-06-25.

No runtime/behaviour change involved — documentation & skill alignment only.