docs: tag README and docs signup CTAs for link attribution

[us]

Why

Every signup link on the repo and on docs.fastcrw.com pointed at a bare
/register. Measured over 30 days on the funnel data, those two surfaces are
the highest-converting entry points we have (github 113 visitors / 17 signups,
docs 57 / 8), but each is a single undifferentiated row: there is no way to tell
which README CTA or which docs page produced them.

The referer header cannot answer it either. Docs referers arrive truncated to
the origin under strict-origin-when-cross-origin (125 hits on
https://docs.fastcrw.com/ against 2 on any deep page), and every README link
reports the same https://github.com/us/crw.

What changed

• README.md and README.zh-CN.md: the signup CTAs carry ?ref=gh-<slot>
  (hero, quickstart, sdk, table, zh).
• docs/index.html: the navbar and TOC CTAs carry ?ref=docs-home-<slot>.
  scripts/build-docs-pages.mjs rewrites docs-home- to the page's own slug
  when generating each page, so docs/scraping/index.html emits
  ?ref=docs-scraping-nav and ?ref=docs-scraping-cta.
• docs/docs/{quick-start,search,monitoring}.md: inline signup links carry
  ?ref=docs-<slug>-inline.
• The 49 generated pages are regenerated to match.

Notes

• Visible link text is unchanged everywhere. The slug lives only in the
  href, including where the anchor text is itself the URL
  ([fastcrw.com/register](...) in quick-start).
• docs/sitemap.xml is deliberately left alone. Regenerating bumped
  lastmod on all 50 URLs, which would report 50 content updates to crawlers
  for a query-string change.
• The install command curl -fsSL https://fastcrw.com/install | sh is
  untouched.
• No behaviour change in the engine; markdown, static HTML and the docs
  generator only.

Follow-up in the private repo

A companion change drops the ref parameter from the address bar on arrival
via history.replaceState. The edge middleware records the full URL
server-side before first paint, so the slug is captured either way and the
reader never sees a tracking string.