Load landing icon size overrides via Book theme custom.scss

[Sendipad]

Motivation

• Homepage SVG icons were rendering oversized because the Book theme compiles its SCSS bundle and previously-applied static CSS rules were not guaranteed to be included in the final theme stylesheet.
• The Book theme exposes a custom import point in themes/book/assets/book.scss, which is the correct extension spot to ensure style overrides are compiled into the active theme CSS.

Description

• Added assets/custom.scss to constrain .feature-icon, .doctype-icon, .usecase-icon and their nested svg sizes so landing icons have a fixed maximum visual size.
• Kept selectors narrow and theme-friendly so the rules are included when the Book theme SCSS pipeline runs (the theme already imports custom).
• No changes were made to markup or icon SVGs; only styling is adjusted to fix visual sizing.

Testing

• Confirmed themes/book/assets/book.scss imports custom so assets/custom.scss will be compiled into the theme CSS (inspection succeeded).
• Verified the new selectors exist in assets/custom.scss using rg and file listing (succeeded).
• Captured a live-page screenshot of https://sendipad.github.io/uph/ using Playwright to validate visual appearance after the change (script ran and saved a screenshot successfully).

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Codex Task