docs: add Hugo and Hextra website

[flexdinesh]

• Owner/result: standalone docs-website/ with a brief homepage and getting-started guide.
• Contracts: isolated module/workspace, pinned Hugo/Hextra, local preview and static build commands.
• Validation/risk: mise run docs:check, mise run check, pre-push/race checks, and local HTTP smoke passed. An existing hook cleanup race reproduced on main; full rerun passed.
• Design: standard Hextra layout; existing code docs and application builds unchanged.
• UI evidence: desktop/mobile layout, search, and mobile navigation verified.