This enhances the `build-docs` utility to allow running it via pnpm and
packaging it as a Nix application. Changes include:
- Added `build` and `cli` scripts to `apps/build-docs/package.json`.
- Implemented a standalone CLI wrapper for documentation generation.
- Added a `trilium-build-docs` package to the Nix flake for use in other projects.
- Integrated `apps/build-docs` into the Nix workspace and build pipeline.
These changes make the documentation build process more portable and easier
to integrate into CI/CD pipelines outside of the main repository.
