Add support for multiple versions to the runtime documentation

[mikir]

Currently, the C++17 runtime documentation does not support switching between the versions.

However, the C++11 runtime documentation does support switching between the Zserio versions.

Switching between the Zserio versions is not suitable for C++17 extension, but we can investigate the possibility of the switching between C++17 extension versions instead.

If this is possible, please consider updating the script scripts/web_pages.sh accordingly.