Document search, filtering, asset operations, and extension points

[wallstop]

The docs site published only an index and a getting-started page, so the long-form guide stayed in README.md and drifted from the code. Four guide pages now carry it, and the drifted README claims are corrected.

Behavior

• Four pages traced to source: search and filtering, asset operations and processors, ordering and persistence, and the runtime extension surface.
• README corrections found while sweeping: the search box is in the window header row and covers every tracked type, arrow buttons move a row to the top or bottom rather than stepping it, Create writes to a per-type folder under the Data Folder, clones strip an existing "(Clone n)" before reapplying one, OR-mode label semantics were omitted, and label editing was missing.

Validation

• Strict site build green on the committed content; red with exit 1 and the target named for a link to a nonexistent page, a missing image, and a page missing from nav.
• npm run lint:llm:full green (19/19 self-test files), npm pack --dry-run unchanged at 172 files, CSharpier clean on 110 files. No C# or Unity change, so the Unity suites are not exercised.

Risk / Rollback

• Risk: low, documentation only. The correction pass restates behavior already in the shipped code; no code path changed.
• Rollback: revert this commit.

Closes the T11 documentation-content items in PLAN.md. Related to #114.

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Note

Low Risk
Documentation-only updates that describe existing editor behavior; no code paths or build artifacts change.

Overview
This PR moves the long-form user guide off README.md into four browsable MkDocs pages—search/filtering, asset operations, organizing/persistence, and extensibility—and wires them into mkdocs.yml nav plus docs/index.md and getting-started.md cross-links. README now points at the published docs site at wallstop.github.io/DataVisualizer/.

README is corrected where it had drifted from the editor: global search lives in the header (all tracked types, field matching, 25-result cap), reorder arrows jump top/bottom (drag is documented), instance actions sit per row, Create uses per-type folders under Data Folder, clone suffix stripping, OR label-filter semantics, UserState persistence details, and new coverage for processors and label editing.

No runtime or editor code changes—documentation and Unity .meta for new markdown only.

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