add conventional Markdown content pages rendered as guide docs

[daveshanley]

Add EnableContentPages support to PrintingPress: discover conventional Markdown files (about.md, quickstart.md, guide.md, auth.md, docs/*.md) next to a local spec and render them as static guide pages alongside the API reference.

• New content pipeline: content_loader.go, content_pages.go and markdown_content_rewrite.go handle discovery, frontmatter, partial expansion ({{< partial >}}) with depth/size budgets, image assets, and link rewriting.
• New render templates for content pages and a content index, plus layout/nav integration (Lit pp-nav guide entries, shared nav cache, CSS) across full and lite bundles.
• Aggregate mode: aggregate_content_pages.go plus discovery/writer updates publish content pages across multi-spec catalogs.
• Config validation now distinguishes URL vs local BasePath/SpecPath, only enabling content discovery for local directories and keeping remote spec metadata out of libopenapi's filesystem BasePath.
• cmd/press flag wiring, pppaths constants, model/catalog additions, and tests for content pages, layout and nav.