Documentation: fix C++ South plugin signatures involving PLUGIN_HANDLE

[mbourhis]

Fix the issue #1069

In the 'plugin_developers_guide/03_south_C_plugins.html' documentation page, the signature of some plugin functions involving PLUGIN_HANDLE is erroneous.

According to the definitions of the function pointers about the South plugins ($FLEDGE_ROOT/C/services/south/include/south_plugin.h), fix these signatures.

And use the appropriate C++ 'cast' operator,
more restrictive and accurate 'cast' operator than the C one, for the 'PLUGIN_HANDLE' object.

[mbourhis]

The code source of the examples have also been updated; see:

• Fix the South plugin API involving PLUGIN_HANDLE fledge-south-random#28
• DHT plugin (https://github.com/fledge-iot/fledge-south-dht): already done in January 2023

[mbourhis]

This PR concerns the 'South plugin' but other documentation pages are also concerned by this 'inconsistency':

• the 'filter' plugins (an issue is already open: Invalid Filter plugin interface #843)
• the 'notification delivery' plugins (https://fledge-iot.readthedocs.io/en/latest/plugin_developers_guide/08_notify_plugins.html)

[mbourhis]

Hi @pintomax, @MarkRiddoch

this PR is about the issue #1069,
where some API plugin signatures involving PLUGIN_HANDLE are erroneous.

So, I am looking for a reviewer for this PR.

Thank you.

[mbourhis]

Hi @MarkRiddoch ,

I would like to know if you are ok or not with this commit
and this improvement in the documentation,
because new users who read the docs (readthedocs.io) -like me- may encounter
this subtlety with the "PLUGIN_HANDLE' as "void *".

Thank you.

[ashish-jabble]

@MarkRiddoch Do we need to merge this into the develop branch?