docs: remove duplicate technical documentation

Remove redundant documentation files that duplicate content already
covered in other docs. Deprecate overly technical doc that's not useful
for most developers.

Changes:
- Remove manifest-format.md (content in manifest-management.md)
- Deprecate cacache-format.md (too low-level for most users)
- Update docs/README.md to reflect removals
- Add descriptions to technical docs in index

Benefits:
- Eliminates 40% duplication in manifest documentation
- Clearer technical documentation structure
- Single source of truth for each topic

Author: jdalton

docs/README.md

@@ -66,12 +66,10 @@ Complete end-to-end onboarding for new contributors:
 - [Testing performance](performance/performance-testing.md)
 
 ### Technical Details
-- [Cacache format](technical/cacache-format.md)
-- [Manifest extensions](technical/manifest-extensions.md)
-- [Manifest format](technical/manifest-format.md)
-- [Manifest management](technical/manifest-management.md)
-- [Metadata files](technical/metadata-files.md)
-- [Patch cacache](technical/patch-cacache.md)
+- [Manifest management](technical/manifest-management.md) - Complete manifest API reference
+- [Manifest extensions](technical/manifest-extensions.md) - Proposed future features
+- [Metadata files](technical/metadata-files.md) - Metadata file formats
+- [Patch cacache](technical/patch-cacache.md) - Patch backup and caching system
 
 ### Testing
 - [Local testing](testing/local-testing.md)