Parameter Descriptor simplification for generic types

Feature request

Feature description

Simplify creation of parameter descriptors for generic types or add documentation to rosdocs (or both).

When trying to mimic behavior in ROS1 for dynamic reconfigure, I noticed there was no documentation on how to set up the parameter descriptors. The interface also ended up being quite strange/esoteric. After trying to find ROS documentation for some time I gave up and looked for sample code which I eventually found here: https://git.3dvisionlabs.com/3dvisionlabs/software/hemistereo/apps/stereo/ros2/-/blob/master/src/hemistereo/src/ros2_wrapper.cpp

I wanted to define a single floating point value with fixed step sizes and min/max values. This ended up with the descriptor declaration:

rcl_interfaces::msg::ParameterDescriptor param_descriptor = {};
param_descriptor.type = rcl_interfaces::msg::ParameterType::PARAMETER_DOUBLE;
// Generalized for lists?
param_descriptor.floating_point_range.resize(1);
param_descriptor.floating_point_range.at(0).from_value = 0.0f;
param_descriptor.floating_point_range.at(0).to_value = 1.0f;
param_descriptor.floating_point_range.at(0).step = 0.001f;
this->declare_parameter<double>("value", 0.5, param_descriptor);

This is quite strange and there is a lot of excess code.

Implementation considerations

While the reason for this being complicated likely comes from the generated messages, that doesn't mean the implementation needs to be messy. It may be reasonable to create convenient tools which allow for the definition of such a variable in much fewer steps:

rcl_interfaces::msg::FloatingPointDescription(args_for_default_description, min, max, step);

This could be a class structure that simply wraps the default constructor and mimics the message types? Regardless of if this is a good idea or not, I think documentation for generic type descriptors should be more available as dynamic_reconfigure's GUI interface is nowhere near as valuable without it.

Start at the declare_parameter entry point and the rcl_interfaces::msg::ParameterDescriptor usage shown in the issue. Compare the proposed convenience wrapper with improving rosdocs for generic type descriptors, then determine which direction is accepted. Done means an agreed API or documentation change that makes floating-point ranges and other descriptors easier to define.