Publish documentation on GitHub Pages

[velo]

Summary

• Replace the old DocBook XML documentation (querydsl-docs/ module) with a modern Jekyll + just-the-docs GitHub Pages site under docs/
• Convert all existing documentation from DocBook XML to Markdown with updated content for the OpenFeign fork
• Add new documentation for R2DBC and Kotlin modules
• Add comprehensive migration guide from the defunct upstream querydsl project
• Update README links to point to the new documentation site
• Add GitHub Actions workflow that auto-deploys on changes to docs/**

What changed

New documentation site (docs/):

• Landing page with supported backends table and quick example
• Tutorials: JPA, SQL, R2DBC (new), MongoDB, Collections, Spatial, Kotlin (new), Scala
• Guides: Creating Queries, Result Handling, Code Generation, Alias Usage
• Troubleshooting guide
• Migration guide from defunct com.querydsl project
• Jekyll config with just-the-docs theme, search, edit-on-GitHub links

Deleted:

• querydsl-docs/ module (DocBook XML, XSL stylesheets, jdocbook config)
• docs Maven profile from root pom.xml
• DocBook build lines from dist.sh

Updated:

• README.md — replaced dead querydsl.com links with openfeign.github.io/querydsl
• Added CONTRIBUTING.md with docs contribution guide

Content updates applied during conversion:

• All Maven examples use maven-compiler-plugin with annotationProcessorPaths instead of the old apt-maven-plugin
• Version references use {{ site.querydsl_version }} (resolves to 7.1)
• GroupId uses {{ site.group_id }} (resolves to io.github.openfeign.querydsl)
• Dropped references to removed modules (JDO, Lucene, Hibernate Search)
• Updated supported backends list (added R2DBC, Kotlin)
• Grammar and clarity improvements throughout

Post-merge setup

After merging, enable GitHub Pages in repository Settings > Pages > Source: GitHub Actions. The site will be available at https://openfeign.github.io/querydsl.

Note

This PR was entirely generated by Claude (claude.ai/code).