Doc: Add plugin_conf_dir, fix pluginconfpath and pluginpath option descriptions

[jrohel]

• Add documentation for the plugin_conf_dir configuration option which supports a list of directories for loading libdnf5 plugin configuration files.
• Fix pluginconfpath and pluginpath option descriptions. The options support only a single directory, not a list of directories.
• Added details about .conf file extension requirement and plugin loading conditions.

[m-blaha]

There seem to be problems with building dnf5 on rawhide. I guess swig 4.4 behaves differently?
Also tests

Failing scenarios:
  dnf/upgrade-from-vendor.feature:67  Upgrade vendorpkg from "Second Vendor" and upgrade dependencies and install new dependency from any repos
  dnf/upgrade-from-vendor.feature:91  Upgrade vendorpkg from "Second Vendor", don't change vendor for upgraded dependencies and install new dependency from any repos

seem to be flaky - they depend on the order of elements in rpm transaction.

But these issues are not caused by this PR.

[ppisar]

If you mean the array offset warning-as-an-error in Ruby bindings, it could be triggered by GCC 16 which is now in Fedora 44.

[m-blaha]

Or gcc, thanks! I did not look into it too closely since the failures are definitely not caused by documentation enhancements :)