Replace link to Clara's repository for doc with our own documentation page

Clara, the command line parser library in Catch2, stopped being a separate project from Catch2 about 3 years ago. Since then it was only developed inside Catch2, while the original upstream repository was made inactive and archived.

Given that Clara-inside-Catch2 has also slowly diverged from the separate version, we should provide its documentation inside the Catch2 repo, and replace the link to upstream docs in the own-main docs with link to our own documentation page for Clara.