Doc: Update Installing clang for QDoc page

* Remove temporal phrases (e.g. 'at this time', 'at the time of
  writing')
* Remove references to Qt Creator's libclang and questions about static/
  dynamic linking in the text.
* Describe LLVM and LLVM_INSTALL_DIR environment variable as
  procedural prerequisites for building QDoc from source.

Task-number: QTBUG-69707
Change-Id: I688714b37721ed1180d500674ca4b271ed257799
Reviewed-by: Martin Smith <martin.smith@qt.io>