docs: delete _deprecated/, prune dead-end redirects, add orphan-page check to CI

[palango]

_deprecated/ held 105 unreferenced copies of live pages that contributors occasionally edited by mistake, and 208 of the 645 redirects in docs.json pointed at another redirect or at a path with no page behind it. Nothing in CI noticed when a page on disk fell out of navigation, which is how 25 pages went orphaned. This PR deletes _deprecated/, repairs the redirect graph so every redirect ends at a live page, and adds the orphan check as a report-only step to the existing docs-validation workflow. Closes #2252.

Changes

1. _deprecated/ is deleted. It had zero references from docs.json or live content. Before deleting, every file was compared against its closest live counterpart: 30 are byte-identical, the other 75 differ only where the live page moved on (fixed links, updated dates, added descriptions) or where the content is retired L1/Alfajores material. Nothing needed porting. No redirects added, since these paths were never reachable.
2. 53 redirect chains are flattened. AGENTS.md says a redirect points at the final page; these pointed at another redirect's source. Includes the thirdweb chain named in the issue.
3. 152 dead-end redirects are re-pointed and 3 removed. The issue left removal vs re-pointing open; re-pointing keeps old inbound links working, so every redirect whose chain ended at a missing page now points at the live page that answers the same question (most had near-exact matches, e.g. /cli/account to /tooling/libraries-sdks/cli/account). The 3 removed entries are the /glossary group: no glossary page exists anywhere, and they formed a redirect cycle. Every new destination exists on disk, is listed in navigation, and is not itself a redirect source.
4. scripts/check-orphans.sh diffs .mdx files on disk against the page paths under navigation in docs.json and exits 1 when it finds orphans. The workflow step has continue-on-error: true, so task: Orphaned-pages audit — re-nav or delete the 22 pages on disk that are missing from navigation #2253 flips it to failing by deleting that single line.

The counts differ from the issue because the redirects drifted since it was measured at bdf40b3; they were re-derived on the branch as the issue asks, and again after the rebase onto #2271, which re-pointed the MiniPay redirects itself.

Verification

bash scripts/check-orphans.sh prints the 25 known orphans (the count #2253 tracks) and exits 1.

npx mintlify broken-links:

success no broken links found

Re-running the redirect analysis after the rebase: 642 redirects, 0 chains, 0 dead ends. The one remaining wildcard with no pages behind it, /developer/rainbowkit-celo/:slug*, predates this PR and is left for #2253.

Notes for #2253

A few re-pointed redirects have an exact-match target among the 25 orphaned pages (the contractkit migration guides, tooling/overview/setup/*), but orphans cannot be redirect targets, so they point at the nearest in-nav page instead. If #2253 adds those pages to navigation, the redirects can be tightened.

[GigaHierz]

Approving after a full verification pass.

• _deprecated/ deletion is safe: the directory is gone and there are zero references to it from docs.json or any live .mdx.
• Redirect graph is sound: across all 632 redirects there are 0 unflattened chains (no destination is itself a redirect source) and 0 internal destinations without a backing live page. Spot-checked the re-pointed classes (e.g. /cli/* → /tooling/libraries-sdks/cli/*) and they resolve.
• The new orphan-page CI step is correctly report-only (continue-on-error: true); scripts/check-orphans.sh runs and honestly lists the existing orphans rather than no-opping.

One pre-existing item unrelated to this PR: the /developer/rainbowkit-celo/:slug* wildcard points at a prefix with no pages — untouched here, fine to handle in the orphan/redirect follow-up (#2253).

[GigaHierz]

Re-approving after the AGENTS.md cleanup (99e84f6) dismissed my prior approval. The only change since my full review is dropping the now-deleted _deprecated/ from the 'Content directories today' list — consistent with this PR deleting the directory. Everything I verified earlier still holds (deletion has zero references, redirect graph has no unflattened chains or missing destinations, orphan CI step is report-only). CI is green.