Use guide index as homepage root

[coisa]

Summary

• remove the template index.html transformation so the guide index document can become the root homepage when guide output is set to .
• document that this template is designed for documentation sets that provide guides and should point guide output to the documentation root
• harden local asset and navigation paths for guide-rendered pages by avoiding fragile base-href behavior and using explicit template paths where needed
• fix search result links so local root pages resolve API links correctly

Validation

• verified the final Twig and template changes against phpDocumentor writer behavior and the generated relative-path logic
• reproduced and fixed the broken search URL concatenation in the template JavaScript
• phpdoc / phpDocumentor CLI is not installed in this environment, so I could not run an end-to-end render locally