docs: clarify logo application on backgrounds

[vitormattos]

Goal

Expand the logo application page with clear visual examples showing which logo version to use on different backgrounds and which applications to avoid.

The visual reference follows a common brand-manual pattern: explicit examples on light backgrounds, dark backgrounds, brand colors, and visually unstable backgrounds, plus incorrect-use examples.

Changes

• Renames the page from "Application" to "Logo on backgrounds".
• Shows correct use on light, dark, and brand-color backgrounds.
• Shows incorrect low-contrast use and white-logo-on-light-background use.
• Adds guidance for photos, textures, and gradients: move the logo to a quiet area or use a solid field.
• Keeps a single content source in manual/main.typ; A4 and 16:9 are still generated from the same source.
• Adapts the grid by format: 2 columns in A4 and 3 columns in 16:9.

Reference

The supplied identity manual explicitly separates "application on backgrounds", "application on unstable backgrounds", and "incorrect uses". This PR brings the same type of practical guidance into LibreSign without copying that manual's visual identity.

Expected verification

• Build both A4 and 16:9.
• Check that cards and logos are not clipped.
• Verify readability of correct/incorrect examples in both formats.
• Preserve PDF/UA-1 and the existing CI checks.

Additional visual refinements

While reviewing the 16:9 output, we found layout inconsistencies on the background-application and correct/incorrect-use pages. This PR now also normalizes card height in slide mode, stabilizes the title/preview areas, keeps the "Partner" crowding example attached to the logo, and removes unnecessary all-caps labels. These refinements are intentionally included here because they directly affect the presentation of the new content while preserving the same single source for A4 and 16:9.