build ruby with --enable-shared for source installs

What does this MR do?

This MR adds --enable-shared to the ruby's configure flags in the docs for the source installation.

This change is rather urgent, since without it ruby is built without shared library support, which prevents source installations to upgrade to yesterday's security releases 13.9.5 or 13.10.1 (I haven't tested 13.8.7).

Fixes: #326662 (closed)

Does this MR meet the acceptance criteria?

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