[clang][docs] Improve "Obtaining Clang" section (#71313)

## Why

The documentation is written relatively to `clang-llvm`, not the root
repository directory. However, some steps in the documentation are
relative to the repository root, which is not correct.

## What
Documentation steps have been modified to make them correct and outdated
ones were updated. Some details:
* Correct paths in documentation
* Change `bootstrap.py` -> `configure.py` since bootstraping Ninja has
[slightly
changed](https://github.com/ninja-build/ninja/tree/master?tab=readme-ov-file#python)