[Flang] Fix release blocker issue #46931 related to documentation.

Summary:
Fixes bug : https://bugs.llvm.org/show_bug.cgi?id=46931
This commit add a new flag -DLLVM_ENABLE_SPHINX=ON to cmake command to generate sphinx documentation,
along with new cmake targets `docs-flang-html`.
`ninja docs-flang-html` - generates sphinx documentation.
Generated release notes are present in <builddir>/tools/flang/docs/html/ folder.

Reviewers: richard.barton.arm, DavidTruby

Tags: #flang

Differential Revision: https://reviews.llvm.org/D85470