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