docs(site): UX, nav, and reference cleanup

[chris-colinsky]

Summary

First of the docs-phase PRs. Site UX, nav, and reference touch-ups; no source changes.

• Homepage: the spec mention is now a standalone section linking to openarmature.org, and the left nav renders on the landing page (was hidden behind hide: [navigation]).
• Header: the "OpenArmature" wordmark is clickable (local override of Material's header partial). The homepage variant also suppresses the default wordmark/page-title scroll swap, which read as a glitch when both texts are identical.
• Concepts nav: added Parallel branches (proposal 0011 landed without a Concepts entry).
• Reference index: added the missing openarmature.prompts bullet, refreshed graph / llm / checkpoint blurbs to mention features that shipped in 0.6.x (parallel-branches, multimodal + structured output, state-migration registry), and dropped a trailer paragraph that just restated Python's underscore convention.

Test plan

• uv run --group docs mkdocs build --strict succeeds
• Homepage: spec section visible below the feature grid; link opens openarmature.org in a new tab
• Homepage: left nav renders
• Clicking "OpenArmature" wordmark from any page returns to the homepage
• On a subpage, scroll animates the wordmark out and the page title in (unchanged from Material default)
• On the homepage, scroll leaves the wordmark in place (no swap)
• Reference index lists openarmature.prompts

[Copilot]

Pull request overview

Docs-site focused PR to improve navigation and header UX in the MkDocs Material site, plus small reference index cleanup, without changing the library source.

Changes:

• Enabled left navigation on the homepage and added “Parallel branches” to the Concepts nav.
• Added a Material header partial override so the “OpenArmature” wordmark links home and remains pinned on the homepage (no scroll-swap glitch).
• Refreshed the reference index blurbs and added the missing openarmature.prompts entry.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
mkdocs.yml	Enables theme overrides and adds Concepts nav entry for parallel branches
docs/stylesheets/extra.css	Styles for clickable wordmark link and homepage header pinning behavior
docs/reference/index.md	Updates reference bullets/blurbs and adds openarmature.prompts
docs/overrides/partials/header.html	Material header override to make wordmark clickable and adjust homepage title swap behavior
docs/index.md	Shows homepage left nav and reworks the spec section content/link

Comments suppressed due to low confidence (1)

docs/overrides/partials/header.html:47

• In this override, config.site_name is inserted into attributes/text without escaping (e.g., aria-label here and the visible wordmark text below). Since this is user-configurable content, it should be HTML-escaped (e.g., apply | e consistently) to avoid broken markup and reduce injection risk.

        <a href="{{ config.extra.homepage | d(nav.homepage.url, true) | url }}" class="md-header__topic md-header__topic-link" aria-label="{{ config.site_name }}">
          <span class="md-ellipsis">
            {{ config.site_name }}
          </span>

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