Add advanced project wiki

[neikiri]

Summary

• Add WIKI.md with an advanced Czech guide for users, administrators, and developers.
• Cover supported conversions, Explorer integration, CLI/headless usage, conversion pipeline, settings, backend behavior, build/release notes, QA matrix, troubleshooting, security, and extensibility.
• Link the wiki from README.md and intentionally avoid adding a repository file-structure section.

Review & Testing Checklist for Human

• Skim WIKI.md for wording, product positioning, and whether the Czech terminology matches your preference.
• Confirm the wiki omits repo file-structure documentation as requested.
• On Windows, use the QA matrix in the wiki as the recommended end-to-end manual verification plan when testing a release build.

Notes

• Local checks passed: cargo fmt --manifest-path /home/ubuntu/repos/xolariq/Cargo.toml --all -- --check, git diff --check, cargo test --manifest-path /home/ubuntu/repos/xolariq/Cargo.toml -p xolariq-core, and cargo clippy --manifest-path /home/ubuntu/repos/xolariq/Cargo.toml -p xolariq-core --all-targets -- -D warnings.
• Full workspace cargo test and cargo clippy --workspace --all-targets were attempted after installing Linux Tauri prerequisites, but the Tauri build script expects missing Linux sidecar binaries such as external/ffmpeg-x86_64-unknown-linux-gnu; the project docs describe Windows sidecar setup as the supported full build path.

Link to Devin session: https://app.devin.ai/sessions/67c3a7ae21b94702a2fec7ef3fda5042
Requested by: @neikiri