chore - add Prismplane docs site and strict docs CI (#9)

[dannymeijer]

Summary

This PR turns the existing IncQL documentation into a strict MkDocs Material site and completes the screenshot-annotated Prismplane landing page. The homepage now explains IncQL as a typed data-logic plane through a distinct physical Prism visual, accessible syntax-surface examples, and a clearer path from authoring to inspection and execution.

Closes #9.

Deployment note: this remains a draft. It does not publish the unfinished site to production; Pages continues to deploy from main only after merge.

Type of change

• Bug fix
• New feature
• Refactor / maintenance
• Documentation
• CI / tooling
• RFC (adds/updates docs/rfcs/*)

Area(s)

Select the primary areas touched (labels sync from checked lines when the triage workflow runs):

• Package & tests
• Specification (RFCs)
• Documentation
• Automation & repo config
• Other

Key details

• User-facing behavior: adds a responsive IncQL launch page with an explicit IncQL/Prism identity distinction, a purpose-built process-flow scene, engine icons, a smaller semantic-model bridge, and five keyboard-accessible authoring-surface tabs showing equivalent Incan, SQL, DataFrame, LazyFrame, and pipeline examples.
• Internals: moves the tab behavior into docs/javascripts/prismplane.js, loads it through MkDocs, adds local Material-style SVG assets, replaces the previous process-flow raster variants with process-flow-stage-v3.png, and preserves a readable progressive-enhancement fallback when JavaScript is unavailable.
• CI and docs platform: adds the Material documentation configuration, navigation, strict documentation checks, link checking, accessibility smoke checks, and the Pages workflow already developed on this branch.
• Risks: the homepage uses custom HTML, CSS, and JavaScript inside MkDocs. Strict builds, responsive browser QA, keyboard checks, progressive-enhancement checks, and the existing package/consumer gates cover the main regression paths.

Testing / verification

• make ci (or make fmt-check, make build, make test)
• Manual verification described below

Manual verification notes:

• make ci: formatting, test-style, vocab-companion tests, 216 registry checks, library build, and 269 IncQL tests passed. Its final public-consumer target initially encountered sandboxed DNS; make smoke-consumer then passed with crates.io access.
• mkdocs build --strict -d /tmp/incql-site-publish
• node --check docs/javascripts/prismplane.js
• git diff --check and git diff --cached --check
• Browser QA at desktop, tablet, and mobile widths: no horizontal overflow; clean console; all five tabs switch examples by click and keyboard; active-tab semantics and no-JavaScript fallback verified; Prism artwork and engine icons remain legible responsively.

Docs impact

• No docs changes needed
• Docs updated
• Docs follow Divio intent (tutorial/how-to/reference/explanation) where applicable

If docs updated:

• Link(s): docs/index.md, mkdocs.yml, docs/stylesheets/prismplane.css, docs/javascripts/prismplane.js

Checklist

• I kept public docs user-focused and moved internals to contributing docs when appropriate
• I avoided duplicating canonical install/run instructions in multiple places
• I added/updated tests where it materially reduces regressions