[docs][RISCV] Use anonymous references in RISCVUsage to avoid warnings

2a5661c8 added a new external link with
the link text "0.2 draft specification". Surprisingly, as multiple links
have this same text but different targets this causes a warning, which
causes a failure on the llvm-sphinx-docs builder (which treats warnings
as errors). As suggested in
<https://github.com/sphinx-doc/sphinx/issues/3921>, this commit moves to
using anonymous references for the links in the experimental extensions
section.