Move docs to kakao.github.io/actionbase and retire actionbase.io #507

Context

We're retiring the actionbase.io domain in a few months. It's currently attached as a custom domain to the GitHub Pages site in kakao/actionbase, so kakao.github.io/actionbase/ redirects to it. Once the domain is gone, every link to actionbase.io (README, issue template, search results, talks, posts) stops working.

GitHub Pages only redirects from github.io to a custom domain, never the other way. Removing the custom domain makes kakao.github.io/actionbase/ canonical, but leaves actionbase.io dead unless something else serves it. DNS for actionbase.io is on ns1/ns2.kakao.com, and the A records already point to GitHub Pages.

Decision

Make https://kakao.github.io/actionbase/ the canonical docs URL. Keep actionbase.io alive for a few months as a redirect, served from a separate Pages repo in the actionbase-io org (actionbase-io/actionbase-io.github.io). Its index.html and 404.html send every path to the same path under kakao.github.io/actionbase/. After that, archive the redirect repo and drop the domain.

Alternatives considered: a server-side 301 on internal infra, which reindexes faster but needs DNS and certificate work outside the project; and cutting over with only an announcement banner, which breaks every existing link on day one.

Consequences

The site moves from / to /actionbase/. Astro doesn't add base to root-relative links in Markdown, so about 110 ](/...) links under website/src/content, the <img> in quick-start.mdx, and the favicon in Head.astro have to become base-aware first. That's a PR with no behavior change on actionbase.io. A second PR then deletes CNAME, updates deploy-website.yml and the default site in astro.config.mjs, and replaces actionbase.io in README.md, website/README.md, llms-txt.mdx, .github/ISSUE_TEMPLATE/config.yml, and the guide app's Layout.tsx.

On cutover day, we remove the custom domain from kakao/actionbase and attach actionbase.io to the redirect repo. There's a short gap between the two steps and until the certificate is issued. If actionbase.io is a verified domain on the kakao org, it has to be removed there first. Afterwards we should verify it on actionbase-io to prevent takeover, which needs a TXT record from the internal DNS team.

The redirect is HTML/JS, not a 301, so search engines will move over more slowly than with a server-side redirect. Paths are preserved, so deep links keep working until the domain is dropped. The repo's About website, Google Search Console, and the GA property also need updating.