Remove docusaurus

[texodus]

Remove Docusaurus and replace with a lightweight custom docs site, and modernizes the feature gallery and example configurations along the way.

• Remove docusaurus, replace with a minimal mdbook-based docs build,custom HTML/TS entry points (index.html, examples.html, block.html),and a simple static file server (server.mjs). While docusaurus isn't speciifically a problem, it is a large dependency (this changes removes 9k loc from the pnpm-lock.yaml, almost half the project's entire dependencies) and Perspective uses very little of its advanced functionality. Also, the numerous React ecosystem dependencies caused implicit workspace version constraints which can now be lifted.
  
• Migrate datagrid plugin_config values in features.ts, move column styling from the legacyplugin_config.columns format to the current top-level columns_configAPI (number_fg_mode, number_bg_mode, fg_gradient, bg_gradient, pos_fg_color/neg_fg_color, pos_bg_color/neg_bg_color).
• Add new feature for label-bar mode, group_rollup_mode: "flat",X/Y Scatter with label field, and number/date/string format options (signDisplay, significantDigits, useGrouping, dateStyle/timeStyle, custom datetime components, bold/italics format mode).
• Gallery montage now renders the gallery as a single montage image instead of individual <img> elements. The build script shuffles feature indices once (shared across light/dark themes) and writes montage_map.json with the tile-to-index mapping. The gallery component fetches this map and translates click coordinates to the correct feature index. Theme toggling swaps between montage_light.png and montage_dark.png.
• Refactor the tools/ project, remove unused scripts (sh.mjs, run_emsdk.mjs, fix_cpp.mjs, emsdk.patch), simplify remaining build helpers.
• Enable doctests, reorganize tests — add to_format/ test directory with data_slice, leaves_only, to_arrow, to_csv, to_json, to_ndjson, and export specs; move existing format tests under the new structure. Adds end-to-end tests for The export function does not support Traditional Chinese. #3152 also.