[analyzer] ClangSA should tablegen doc urls refering to the main doc page

AFAIK we should prefer
https://clang.llvm.org/docs/analyzer/checkers.html to
https://clang-analyzer.llvm.org/{available_checks,alpha_checks}.html

This patch will ensure that the doc urls produced by tablegen for the
ClangSA, will use the new url. Nothing else will be changed.

Reviewed By: martong, Szelethus, ASDenysPetrov

Differential Revision: https://reviews.llvm.org/D121387