Enforce unique doc filenames and simple wiki-links (#109)

## Summary
- Rename section index files to match their titles (tutorials.md, reference.md, how-to.md, explanation.md) so all filenames are unique
- Convert all ~47 path-based wiki-links to simple filename format across 15 files
- Update doc-filenames task to no longer skip index.md files
- Update doc-links task to reject path-based links containing '/'

This ensures all wiki-links work correctly in obsidian.nvim by making links resolvable by filename alone.

## Testing
- `mise run doc-filenames` - all unique
- `mise run doc-links` - no broken or path-based links
- `mise run doc-titles` - no duplicates

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/109

docs/reference/services/devpi.md

@@ -32,6 +32,6 @@ Root password stored in 1Password (blumeops vault), injected via ExternalSecret.
 
 ## Related
 
-- [[how-to/use-pypi-proxy]] - Client configuration and package uploads
+- [[use-pypi-proxy]] - Client configuration and package uploads
 - [[argocd]] - Deployment
 - [[1password]] - Secrets management