feat: fix wrong path prefixes when using multiple docsDirs

[puehringer]

Summary

Fixes multiple bugs where docsDir configured as an array of sections only worked correctly for the first section. Root cause: the plugin collapsed multi-section config down to a single docsDir string internally and reused it everywhere instead of resolving each file's own section.

• Wrong URLs for non-first sections - when a section's filesystem path differs from its routeBasePath, only the first section had its path correctly stripped from generated URLs; every other section leaked its raw directory into the output.
• Same-named files across sections collided - a file with the same name in two sections could resolve to the same route, silently dropping one file's link/output in favor of a -2 suffixed duplicate in the wrong section.
• Blog routeBasePath was hardcoded - the plugin assumed the blog always lives at blog, breaking output when presets.blog.routeBasePath is customized. Added blogDir/blogRouteBasePath options (defaulting to the previous behavior) to make this configurable.

Testing

Added regression tests covering all three scenarios; verified they fail on the pre-fix code and pass after. Existing test suites still pass - no behavior change for single-section (string docsDir) configs.

[rachfop]

Merged onto main in 7e29899 (squash, conflict resolution against #61/#62/#63 done on my side — README section retargeted into docs/configuration.md). Your multi-docsDir fixes are live: per-section URL prefixing, per-section route scoping (no more same-name collisions), and the blogDir/blogRouteBasePath options. Thank you — solid diagnosis of the first-section collapse.

[rachfop]

Merged as 7e29899 (squash-merged onto main with attribution to @puehringer) — the fork declined maintainer pushes, so the branch's content was landed manually after resolving conflicts with the recent README split and absolute-slug fixes. The per-file section resolution, section-scoped routes, and blogDir/blogRouteBasePath options are all live on main, covered by the test suite, and documented in docs/configuration.md. Thanks for the thorough fix!