File addition .rooignore (#15414)

This PR introduces a `.rooignore` file to the root of the repository to
optimize how AI coding assistants (like Roo) interact with the RAGFlow
codebase.

Currently, when AI agents index the workspace, they can waste tokens and
processing time reading through generated files, caches, large
dependency artifacts, and runtime logs. This `.rooignore` file provides
a standard configuration to exclude these irrelevant directories and
files (such as `.venv/`, `node_modules/`, `__pycache__/`, logs, and
large binaries). This significantly reduces indexing noise, prevents
accidental reads of sensitive or bulky local data, and ensures AI coding
agents remain focused strictly on relevant source code.

### Type of change

- [x] Other (please describe): Developer Experience (DX) / AI Tooling
configuration