[SYCL][Doc] Update version of python required to build the llvm (#6293)

* Correct version of Python that is required for LLVM build
* Fix several additional issues on the page

Author: fineg74

sycl/doc/GetStartedGuide.md

@@ -37,7 +37,7 @@ and a wide range of compute accelerators such as GPU and FPGA.
 
 * `git` - [Download](https://git-scm.com/downloads)
 * `cmake` version 3.14 or later - [Download](http://www.cmake.org/download/)
-* `python` - [Download](https://www.python.org/downloads/release/python-2716/)
+* `python` - [Download](https://www.python.org/downloads/)
 * `ninja` -
 [Download](https://github.com/ninja-build/ninja/wiki/Pre-built-Ninja-packages)
 * C++ compiler
@@ -46,7 +46,7 @@ and a wide range of compute accelerators such as GPU and FPGA.
   * Windows: `Visual Studio` version 15.7 preview 4 or later -
     [Download](https://visualstudio.microsoft.com/downloads/)
 
-Alternatively, you can use Docker image, that has everything you need
+Alternatively, you can use Docker image, that has everything you need for building
 pre-installed:
 
 ```
@@ -316,6 +316,7 @@ the following tools need to be installed:
 
 * doxygen
 * graphviz
+* sphinx
 
 Then you'll need to add the following options to your CMake configuration
 command:
@@ -520,7 +521,7 @@ python $DPCPP_HOME/llvm/buildbot/check.py
 ```bat
 python %DPCPP_HOME%\llvm\buildbot\check.py
 ```
-
+Make sure that psutil package is installed.
 If no OpenCL GPU/CPU runtimes are available, the corresponding tests are
 skipped.