refactor: split forms/general into elements, layout, validation; add wizard

forms/general.html had grown into a 623-line monolith containing input
examples, layout patterns, and validation demos all on one page. Splits
it into three focused pages plus a brand-new wizard:

- forms/elements.html    Quick example, input groups, checks /
                         radios / switches, selects (single + multi) /
                         range / file / color, floating labels,
                         disabled / readonly
- forms/layout.html      Horizontal form, inline form (new), different
                         heights, different widths
- forms/validation.html  Custom .needs-validation pattern, browser-
                         default validation, .needs-validation-tooltip
                         variant (new). Shared submit-handler script.
- forms/wizard.html      Four-step wizard with per-step validation,
                         password-match check, animated step indicator
                         (CSS counters + pseudo-elements), and a final
                         review-and-confirm summary.

Drops the original forms/general.astro -- this is RC, so no stable
deprecation cycle required. Sidenav update lands separately.