Refactor styles and add ArcRendering component (#421)

[pkd2512]

What's in this pull request

Refactors the SCSS reset and typography layers and introduces the new ArcRendering component. Addresses issue #421 and follows on from #420.

Style refactoring (src/scss/reset/_typography.scss, src/scss/reset/_main.scss, src/scss/main.scss):

• Replaced hardcoded #404040 with the $theme-colour-text-primary design token for consistent theming.
• Made the SVG text selector explicit (svg text) and documented intent — it now clearly targets chart/visualization text rather than colliding with the HTML namespace.
• Removed redundant display: block declarations on p, ol, ul (block-level by default).
• Consolidated cascade fragmentation: nested li::marker inside li using &::marker; grouped list-style-type rules and made ol explicit.
• Standardized @include ordering (mixins → properties → nested selectors) across .theme, kbd, and the typography root.
• Added clarifying comments on the 0.9 font-size multiplier and the nested-blockquote pattern.
• Moved font-smoothing declarations from scoped to root-level for global coverage.

New ArcRendering component (src/components/ArcRendering/):

• Component, stories and MDX docs for rendering Arc-shell pages.
• Article updated to conditionally skip the <main> wrapper when inside an Arc shell.
• Storybook Wrapper updated to support the new rendering path.

Before submitting, please check that you've

• Read our contributing guide
• Documented any new components or features (ArcRendering.mdx)
• Tagged an editor to review this PR

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pnpm add https://pkg.pr.new/@reuters-graphics/graphics-components@422

commit: 3cbd47f