docs(l1,l2): document libclang requirement

[Oppen]

Due to use of bindgen, RocksDB requires libclang to compile.
This was previously undocumented.

[Copilot]

Pull Request Overview

Document libclang as a prerequisite to build from source because RocksDB depends on bindgen, which requires libclang.

• Add libclang to the prerequisites list
• Note its association with RocksDB

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[Copilot]

[nitpick] The prerequisite is underspecified and links to a generic Clang docs page. Please clarify that the libclang development package is required, link to bindgen’s requirements (the actual reason for the dependency), and give brief install hints and the LIBCLANG_PATH note.

Suggested change

	- [libclang](https://clang.llvm.org/docs/index.html) (for RocksDB)
	- **libclang development package** (required by [bindgen](https://rust-lang.github.io/rust-bindgen/requirements.html), which is used by RocksDB)
	- On **Debian/Ubuntu**: `sudo apt install libclang-dev`
	- On **Fedora**: `sudo dnf install clang-devel`
	- On **macOS**: `brew install llvm` (and see [bindgen requirements](https://rust-lang.github.io/rust-bindgen/requirements.html#macos))
	- If libclang is not found automatically, set the `LIBCLANG_PATH` environment variable to the directory containing `libclang.so`/`libclang.dylib`.