feat: relegate legacy granting guides to a Low-Level APIs section

[sybohy]

Why

Part 2 of the Access Grants front-door refactor (#1114, merged). Access Grants are GA and recommended; the device-level Smart Locks and ACS APIs are the low-level surfaces and should no longer dominate the main nav.

What

The entire Smart Locks and Access Control Systems capability sections move to a new "Low-Level APIs" group at the very bottom of the left nav. Both trees keep their exact internal structure from before (access codes under Smart Locks, credential management under ACS) — the group's top-level entries are simply Smart Locks and Access Control Systems, no extra nesting.

## Low-Level APIs
* Smart Locks
  * Get started / Lock & Unlock / Access Codes (full original tree)
* Access Control Systems
  * Quick Starts / Connect an ACS / Users / Credentials / Encoders / Troubleshooting (full original tree)

• ⚠️ Warning banners on both section overviews: these are low-level APIs — use Access Grants for granting access; discuss your use case with the Seam team (support@seam.co) before building on them. No deprecation commitment anywhere.
• ✅ Pointer hints on the granting-relevant sub-pages (access codes, managing credentials, ACS quick starts, ACS user management)
• 🔀 .gitbook.yaml redirects for all 38 moved pages + retargeted pre-existing redirects that pointed into the moved trees
• Repo-wide inbound-link updates; cross-section relative links fixed by depth

Validation

• validate-paths / validate-links / validate-orphan-pages / validate-redirects — all ✅
• Absolute docs.seam.co URLs in the auto-generated api-reference resolve via the new redirects (validator is redirect-aware)

🤖 Generated with Claude Code

[sybohy]

Updated per Sy's review: removed the "expect to deprecate in the medium term" wording (no product sign-off, and it contradicted the GA changelog's "fully supported"). The banners now position these as low-level APIs — use Access Grants for granting access, and discuss your use case with the Seam team (support@seam.co) before building on them. No deprecation commitment anywhere in this PR now.

[sybohy]

Rebuilt per Sy's review: the whole Smart Locks and ACS sections now move under Low-Level APIs (not just the granting sub-trees), with their original internal structure intact and no extra nesting level — top-level entries are Smart Locks and Access Control Systems. 38 redirects in place; all validators green. Banner wording remains the soft "discuss with the Seam team" version.