docs: fix broken site images + in-body links + Slack app-install wording (#90)

[isadeks]

Closes #90.

Problem

#90 reported the Slack setup screenshots 404 on the published docs site. Investigating surfaced two related docs-site issues:

1. Missing images (the reported bug). The sync script rewrites ../imgs/foo.png → /<base>/imgs/foo.png (served from docs/public/), but it never copied docs/imgs/ into docs/public/imgs/. The two pre-existing site images had been hand-copied; the four Slack screenshots were added to docs/imgs/ only, so they 404'd.
2. Broken in-body design-doc links, site-wide. rewriteDocsLinkTarget emitted bare root-relative routes like /architecture/orchestrator with no base prefix. The site is served under base: /sample-autonomous-cloud-coding-agents, so those links resolved to the domain root and 404'd. (Starlight prefixes its own nav links automatically; our rewritten markdown-body links weren't.) This is what made the blueprint paragraph's ARCHITECTURE.md / REPO_ONBOARDING.md links dead.

Also reworded the Slack app-install prerequisite, which suggested using a personal workspace to get around corporate app-install restrictions — now points users at their Slack admin instead.

Fix

• copyAssetDir step in sync-starlight.mjs mirrors source imgs/ into public/imgs/, so every committed image is published (and future images won't regress). The four Slack PNGs are committed.
• Base-prefix every rewritten in-body link with docsBase — the same prefix the image rewrite already applies. Re-synced all mirrors; the bulk of the diff is purely the base-path prefix added to existing links (verified: no content changes).
• Reword the Slack app-install prerequisite.

Verification

Verified against a local astro preview (serves the exact dist/ output that ships to GitHub Pages, base path included):

• All four Slack images return HTTP 200 at /sample-autonomous-cloud-coding-agents/imgs/... and render inline on the Slack guide.
• Orchestrator in-body design-doc links now carry the base prefix; target pages (repo-onboarding, architecture) return 200.
• yarn build → 62 pages, clean.

[ayushtr-aws]

Verified locally. Minor nit: (dead startsWith('#') guard)

I traced every return path of rewriteDocsLinkTarget():

• all undefined, or
• a route from developerGuideAnchorRoutes / userGuideAnchorRoutes (start with /), or
• explicitGuideRoutes[stem] + anchorSuffix (starts with /), or
• /architecture/${slug}${anchorSuffix} (starts with /).

No non-undefined return can ever start with # — anchors are always appended as a suffix to a /… path. So rewritten.startsWith('#') ? rewritten : … is genuinely unreachable, and the comment ("Anchors #… stay untouched") is slightly misleading since a bare anchor returns undefined earlier. This one stands as written — harmless dead defensive code.

[isadeks]

@ayushtr-aws thanks for the review! Addressed your nit in 041392e: removed the unreachable rewritten.startsWith('#') guard and rewrote the comment to explain why the prefix always applies (every non-undefined return from rewriteDocsLinkTarget is a /… route; bare #… anchors and external links return undefined and keep their original text). Verified the sync output is byte-identical — pure dead-code removal, no behavioral change. Mind re-approving? The new commit dismissed your approval.