[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Documentation steps have been modified to make them correct and outdated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)

Author: robincaloudis

clang/docs/LibASTMatchersTutorial.rst

@@ -22,7 +22,7 @@ started guide <https://llvm.org/docs/GettingStarted.html>`_.
 
 .. code-block:: console
 
-      cd ~/clang-llvm
+      mkdir ~/clang-llvm && cd ~/clang-llvm
       git clone https://github.com/llvm/llvm-project.git
 
 Next you need to obtain the CMake build system and Ninja build tool.
@@ -33,11 +33,11 @@ Next you need to obtain the CMake build system and Ninja build tool.
       git clone https://github.com/martine/ninja.git
       cd ninja
       git checkout release
-      ./bootstrap.py
+      ./configure.py --bootstrap
       sudo cp ninja /usr/bin/
 
       cd ~/clang-llvm
-      git clone git://cmake.org/stage/cmake.git
+      git clone https://gitlab.kitware.com/cmake/cmake.git
       cd cmake
       git checkout next
       ./bootstrap
@@ -50,7 +50,7 @@ Okay. Now we'll build Clang!
 
       cd ~/clang-llvm
       mkdir build && cd build
-      cmake -G Ninja ../llvm -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -DCMAKE_BUILD_TYPE=Release -DLLVM_BUILD_TESTS=ON
+      cmake -G Ninja ../llvm-project/llvm -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -DCMAKE_BUILD_TYPE=Release -DLLVM_BUILD_TESTS=ON
       ninja
       ninja check       # Test LLVM only.
       ninja clang-test  # Test Clang only.
@@ -65,7 +65,7 @@ Finally, we want to set Clang as its own compiler.
 .. code-block:: console
 
       cd ~/clang-llvm/build
-      ccmake ../llvm
+      ccmake ../llvm-project/llvm
 
 The second command will bring up a GUI for configuring Clang. You need
 to set the entry for ``CMAKE_CXX_COMPILER``. Press ``'t'`` to turn on
@@ -89,7 +89,7 @@ live in the ``clang-tools-extra`` repository.
 
 .. code-block:: console
 
-      cd ~/clang-llvm
+      cd ~/clang-llvm/llvm-project
       mkdir clang-tools-extra/loop-convert
       echo 'add_subdirectory(loop-convert)' >> clang-tools-extra/CMakeLists.txt
       vim clang-tools-extra/loop-convert/CMakeLists.txt
@@ -314,7 +314,7 @@ handiwork:
 
 .. code-block:: console
 
-      cd ~/clang-llvm/llvm/llvm_build/
+      cd ~/clang-llvm/build/
       ninja loop-convert
       vim ~/test-files/simple-loops.cc
       bin/loop-convert ~/test-files/simple-loops.cc