docs: replace offline deployments terminology to air-gapped (#19625)

This PR comprehensively updates the offline deployments documentation to
use more precise "air-gapped" terminology and improves consistency
throughout the documentation.

## Changes Made

### Terminology Updates
- **Title**: Changed from "Offline Deployments" to "Air-gapped
Deployments"
- **Summary**: Updated to prioritize "air-gapped" terminology and added
"disconnected" to cover additional deployment scenarios
- **Content**: Updated tutorial references to use "air-gapped" instead
of "offline"
- **Section headers**: 
  - Changed "Offline container images" to "Air-gapped container images"
  - Changed "Offline docs" to "Air-gapped docs"
- **Table headers**: Changed "Offline deployments" to "Air-gapped
deployments"

### Navigation & URL Structure
- **Navigation title**: Updated `docs/manifest.json` to show "Air-gapped
Deployments" in sidebar
- **Navigation description**: Updated to "Run Coder in air-gapped /
disconnected / offline environments"
- **File rename**: `docs/install/offline.md` → `docs/install/airgap.md`
for consistency
- **URL change**: `/install/offline` → `/install/airgap`
- **Subsection anchors**:
  - `/install/offline#offline-docs` → `/install/airgap#airgap-docs`
- `/install/offline#offline-container-images` →
`/install/airgap#airgap-container-images`

### Internal Links & References
Updated all internal documentation links:
- `docs/admin/integrations/index.md`
- `docs/admin/networking/index.md`
- `docs/changelogs/v0.27.0.md` (including anchor reference)
- `docs/tutorials/faqs.md`

### Backward Compatibility
- **Redirects**: Added `docs/_redirects` with 301 redirects:
  - `/install/offline` → `/install/airgap`
  - `/install/offline#offline-docs` → `/install/airgap#airgap-docs`
- `/install/offline#offline-container-images` →
`/install/airgap#airgap-container-images`
- **Content**: Maintains "offline" in the description for broader
understanding
- **Deep links**: All subsection anchors redirect properly to maintain
existing bookmarks

## Rationale

- **"Air-gapped"** is more precise and commonly used in
enterprise/security contexts
- **"Disconnected"** covers additional scenarios where networks may be
temporarily or partially isolated
- **Consistency** ensures filename, URL, navigation, content, and
subsection anchors all align with the same terminology
- **Backward compatibility** maintained through comprehensive redirects
to prevent broken links at any level

## Testing

- [x] Verified all internal links point to the new URL structure
- [x] Confirmed navigation title updates correctly
- [x] Ensured content accuracy is maintained
- [x] Added redirects for backward compatibility (main page +
subsections)
- [x] Updated all cross-references in related documentation
- [x] Verified subsection anchor redirects work properly
- [x] Confirmed no unnecessary .md file redirects

## Result

Complete terminology consistency across:
- ✅ Page title and headers
- ✅ Navigation and breadcrumbs  
- ✅ File names and URL structure
- ✅ Internal documentation links
- ✅ Table headers and section titles
- ✅ Subsection anchors and deep links
- ✅ Backward compatibility via comprehensive redirects

---------

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>
Co-authored-by: david-fraley <67079030+david-fraley@users.noreply.github.com>

docs/tutorials/faqs.md

@@ -332,7 +332,7 @@ References:
 ## Can I run Coder in an air-gapped or offline mode? (no Internet)?
 
 Yes, Coder can be deployed in
-[air-gapped or offline mode](../install/offline.md).
+[air-gapped or offline mode](../install/airgap.md).
 
 Our product bundles with the Terraform binary so assume access to terraform.io
 during installation. The docs outline rebuilding the Coder container with