feat: add developer documentation as new Antora module

[DeepDiver1975]

Summary

• Converts the developer-focused docs from owncloud/ocis (docs/ folder) into a new developer Antora module within this repo
• Source was Hugo/Markdown; 212 pages converted to AsciiDoc with Hugo shortcodes mapped to AsciiDoc equivalents
• Navigation defined in modules/developer/partials/nav.adoc and registered in antora.yml

What's included

• oCIS core: configuration, metrics, migration, backup, release notes/roadmap
• Development guides: getting started, build, testing, debugging, profiling, tracing, CI, extensions, unit testing
• Architecture Decision Records: ADR 0001–0028
• Architecture: upload processing, stat polling, protocol changes, PosixFS, collaborative storage, services communication
• Flow documentation: login, public upload, request flows
• Storage: terminology, spaces, namespaces, drivers, backends (CephFS, EOS, DCFS NFS)
• APIs: HTTP (Graph, WebDAV, TUS upload, authorization) and gRPC
• CLI commands: creating and documenting service-dependent and service-independent CLIs
• Clients: rclone (WebDAV sync with basic auth and OIDC)
• Services (38 services): configuration docs and service-specific guides
• Releasing guide and deployment (developer-focused examples)
• 24 images (SVG diagrams and PNGs) preserved under modules/developer/images/

Conversion notes

• Hugo {{< hint >}} → AsciiDoc admonition blocks ([NOTE], [WARNING], [CAUTION])
• Hugo {{< figure >}} → image:: macros with corrected paths relative to modules/developer/images/
• Hugo {{< include file="..." >}} referencing auto-generated _includes/ files are marked with // AUTO-GENERATED CONTENT: comments — these would need wiring up separately
• _index.md → index.adoc throughout
• Internal cross-references converted from {{< ref >}} to xref:

Test plan

• Build locally with npx antora site.yml and verify the developer module renders
• Check navigation structure in the rendered site
• Review spot-check pages for conversion quality (headings, links, admonitions, images)
• Decide whether to wire up the auto-generated service configuration includes

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