fix: correct mkdocstrings path configuration for local development (#2155)

The mkdocstrings plugin was configured with 'paths: [src]' but the
actual source code is located at 'ragas/src' relative to the mkdocs.yml
file.

This was causing build failures with errors like:
- 'ragas.cache could not be found'
- 'ragas.embeddings could not be found'

The fix updates the path to 'ragas/src' which allows mkdocstrings to
properly generate API documentation when running 'mkdocs serve' locally.

Resolves mkdocstrings module import errors during local documentation
development.

Author: sanjeed5