Broken web link to boost::process documentation

[nigels-com]

This link works:
https://www.boost.org/doc/libs/1_87_0/doc/html/process.html

This link does not:
https://www.boost.org/doc/libs/1_88_0/doc/html/process.html

This page works, but points to the above broken link
https://www.boost.org/doc/libs/1_88_0/

[nigels-com]

Also broken, via the gituhb landing page:
https://www.boost.org/libs/process

[nigels-com]

Also broken, via the README:
https://www.boost.org/doc/libs/develop/doc/html/process.html

[sdarwin]

This PR switched to Asciidoc #443

The top level index.html file https://github.com/boostorg/process/blob/develop/index.html needs to be updated.

The docs can be reached here: https://www.boost.org/doc/libs/1_88_0/libs/process/doc/html/index.html

[klemens-morgenstern]

Thanks for reporting