docs: add mkdocs configuration and API documentation structure

- docs(mkdocs): add mkdocs.yml with material theme and plugin configuration
- docs(mkdocs): configure navigation for core, html, and pdf modules
- docs(docs): add documentation root and homepage
- docs(docs): add core contracts documentation pages
- docs(docs): add html implementation documentation pages
- docs(docs): add pdf implementation documentation pages
- docs(docs): wire mkdocstrings directives for API reference rendering

docs/pdf/client.md

@@ -0,0 +1 @@
+::: omniread.pdf.client