[Documentation] fix invalid links in documentation (#76502)

Author: hanickadot

llvm/docs/SphinxQuickstartTemplate.rst

@@ -170,7 +170,7 @@ Generating the documentation
 
 You can generate the HTML documentation from the sources locally if you want to
 see what they would look like. In addition to the normal
-`build tools <docs/GettingStarted.html>`_
+`build tools <GettingStarted.html>`_
 you need to install `Sphinx`_ and the necessary extensions
 using the following command inside the ``llvm-project`` checkout:
 

llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl08.rst

@@ -122,7 +122,7 @@ Configuring the Module
 
 We're now ready to configure our module, to specify the target and
 data layout. This isn't strictly necessary, but the `frontend
-performance guide <../Frontend/PerformanceTips.html>`_ recommends
+performance guide <../../Frontend/PerformanceTips.html>`_ recommends
 this. Optimizations benefit from knowing about the target and data
 layout.