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 214 of the 635 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. 49 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. 162 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.

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 last commit: 632 redirects, 0 chains, 0 dead ends.

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.