Updated docs theme colours and add reference to highlight and tertiary

[zoharma]

Correct MUI role calls, regroup and introduce brand colours

• Corrected mislabeled MUI palette calls in the Neutral colours reference (e.g. onSurfaceMuted → text.muted) to match the real roles in DiamondDSTheme.ts.
• Split "Intent colours" into two groups: functional MUI-exposed roles (primary, secondary, info, danger, warning, success) and a new "Core brand colours" section (brand, tertiary, highlight). Only brand is still an actual MUI palette role.
• Added a contrastText row to each intent/brand family, previously undocumented.

p.s. We can name Tertiary to something else like Brand Accent if we thing it would be helpful

[zoharma]

Followng @gfrn's comment, I found a few discrepancies that were fixed as part of this PR:

• --ds-action-disabled was only used for disabled Checkbox/Radio icons, so rename it to --ds-control-disabled to better reflect its scope.
• Document --ds-on-surface-disabled as backing both text.disabled and action.disabled
• Document --ds-surface-disabled as action.disabledBackground
• Fix the incorrect “On Surface Disabled” swatch
• Show actual MUI palette paths on intent/brand swatches (for example, primary.main and error.main)
• Rename “{role} Contrast Text” to “On {role}” for consistency