feat(website): quickstart guide with hero hint and bottom CTA

[montfort]

Summary

Adds a standalone /quickstart page and two homepage surfaces that lead to it:

• Hero hint — one subtle line under the install command: "…then follow our short quickstart guide →". Doesn't compete with the existing CTA buttons; just signals there's a guide.
• Bottom "Ready to try it?" section — last section on the landing, single big CTA button to /quickstart. Targets the visitor who reached the bottom of the page without quitting.

The /quickstart page is one long MDX with six sections, designed for a developer who has decided to try the tool and wants the shortest end-to-end path:

1. Install & verify — curl one-liner, straymark --version, update commands.
2. Initialize your repo — straymark init, the .straymark/ tree explained.
3. Your first Charter — CLI flag and /straymark-charter-new skill, the four template sections.
4. AILOG when you commit — when to write one, what goes in, commit-message convention.
5. Status & validate — straymark status, straymark validate --staged, a two-line pre-commit hook.
6. Run your first external audit — the 3-phase, 3-skill multi-model audit cycle.

~10–15 min of reading, heavy on copy-paste-able commands, deliberately not docs-depth. The right-side TOC is Docusaurus' native one.

Why

The site reads at two speeds: people who skim (Hero + workflow blueprint + features grid) and people who decide to try it (Hero install + this guide). Until now there was nothing for the second group — they had to land in the docs and assemble the happy path themselves. The guide is the missing bridge between "I'll try it" and "I have a closed Charter".

Asciinema demo is a separate, follow-up PR (different purpose: a "wow" surface for visitors who haven't decided yet).

i18n

• The five new homepage strings (hero.quickstartHint, hero.quickstartHint.link, gettingStarted.title, gettingStarted.body, gettingStarted.cta) are translated to es and zh-CN.
• The /quickstart.mdx page itself is English-only in this PR; Docusaurus falls back gracefully under /es/quickstart and /zh-CN/quickstart (visitor sees English content inside their localized chrome). Translation of the guide is a deliberate follow-up.

Test plan

• npm run build succeeds for all three locales (verified locally — build/quickstart.html, build/es/quickstart.html, build/zh-CN/quickstart.html exist).
• On /, /es/, /zh-CN/: hero shows the subtle quickstart link under the install command; bottom section shows the CTA in the correct language.
• Clicking either lands on the quickstart page, which renders with Docusaurus' native right-side TOC for the six section headings.

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