[DOCS]: Polish docs site for public launch

[spencrr]

Description

Builds on #47 to upgrade the mkdocs-material site from "functional" to "launch-ready" ahead of going public at microsoft.github.io/RAMPART.

Site identity & metadata

• NEW SVG: Site logo + favicon wired to docs/images/RAMPART.svg.
• Centered hero logo on the home page - replaces inline <div style> with a CSS class (.hero-logo in custom.css) for theming consistency.
• copyright footer added; Material "Made with…" attribution suppressed (extra.generator: false) - Microsoft OSS convention.
• extra.social GitHub link in footer.
• Per-page description frontmatter on all section landing pages and index.md for better SEO and richer search snippets.
• 404 page (docs/404.md) with branded copy + search; GitHub Pages serves it automatically for missing paths.

Navigation & UX

• Header tabs for top-level sections (navigation.tabs + tabs.sticky).
• Breadcrumbs (navigation.path) on every non-root page.
• Instant SPA-style nav + progress bar (navigation.instant, navigation.instant.progress).
• Prev/next page footer on every page (navigation.footer).
• Scroll-tracking URLs for deep-linkable headings (navigation.tracking).
• Edit-this-page pencil that opens the GitHub source

Search

• search.suggest (inline autocomplete), search.highlight (highlight matches on landed page), search.share (share-this-query button).

Content authoring features

• Code annotations (content.code.annotate) - numbered callouts in the Quickstart Step 2 adapter/session example and the Authoring Tests session example, replacing post-block bullet lists. Big pedagogical win.
• Line numbers + click-to-select (content.code.select + optin for linenums="1") on long reference implementations.
• Collapsible details (pymdownx.details) - _FileInjection reference implementation tucked into a ??? note block to keep the surrounding tutorial scannable.
• Platform tabs (pymdownx.tabbed + content.tabs.link) with Linux / macOS and Windows (PowerShell) for venv activation and ad-hoc env-var setting. Tab labels match exactly so selection syncs site-wide.
• Task lists (pymdownx.tasklist) - release-readiness pre-flight in release-process.md now renders as checkboxes; the PR-template checklist on pull-requests.md picks up the styling automatically.
• Acronym tooltips (abbr + pymdownx.snippets + content.tooltips) - site-wide definitions for RAMPART/security-specific acronyms (XPIA, PyRIT, CVE, OG, OIDC, OWASP, SemVer, PyPA, gRPC, SUT). Defined once in docs/_includes/abbreviations.md; auto-appended to every page.

Build hardening

• Strict validation block - omitted_files, absolute_links, anchors configured at warn (fatal under --strict); unrecognized_links at info to stage warnings without blocking until the corpus is fully cleaned.
• not_in_nav for 404.md; exclude_docs for _includes/ and _overrides/ so they don't appear in nav, search, or build output.

Theme overrides

• New docs/_overrides/main.html with an announce banner ("RAMPART is in alpha - APIs may change before 1.0") + announce.dismiss feature. The banner re-appears for everyone when we change its content, so flipping to a "1.0 stable" message after launch is one-line.
• theme.custom_dir: docs/_overrides enables the override.

Plugins added

• privacy - inlines all external assets (Roboto + Roboto Mono fonts, Material icons, Twemoji SVGs) at build time. Deployed site makes zero third-party network calls.
• git-revision-date-localized - adds "Last update: 2 days ago"

Follow-ups (intentionally out of scope)

• Social cards plugin (mkdocs-material[imaging] + Cairo system deps) - adds rich OpenGraph preview images. Deferred to a separate PR to keep this one focused; nothing here blocks adding it later.
• Custom hero landing page (overrides/home.html with CTA buttons + a feature grid) - bigger lift; better as its own "launch polish" PR.

Checklist

• pre-commit run --all-files passes
• Tests added or updated for changes - N/A, docs-only
• Documentation updated
• uv run mkdocs serve --strict compiles