docs: update brand colors/fonts and trim getting-started table

[HAPPYS1NGH]

Follow-up to #7 (docs: restructure navigation and refresh content, already merged).

Screenshots (before → after)

⏳ Pending — to be added once browser capture is available.

• Theme (any page): before = green palette (#15803D/#07C983) · after = blue palette (#5876F4/#7B93F7/#3D5BEB) with Inter body / Space Grotesk headings.
• Getting started — model comparison table: before = 4-column header (Model, Best for, Cost to issue, Ownership model, Trust assumptions) with the last column already empty in every row · after = the orphaned "Trust assumptions" header removed.

Changes

• Switched theme colors to the new blue palette and added Inter/Space Grotesk fonts — refreshed brand look — matches updated brand guidelines.
• Removed the unused "Trust assumptions" table header on getting-started.mdx — the column had no data in any row — table header now matches its content.

Notes

• node scripts/check-docs.mjs, mint broken-links, and mint validate all pass on this branch.
• A few other in-progress pages (neobanks overview page, payment-app guide, ENS-compatible-naming-service architecture page) are intentionally left out of this PR — still incomplete, not part of this change.