Add API and document functionality for frameworks to introspect dependencies and watchers

[maximlt]

While extending Param's README, I realized there's a gap in Param's functionality and documentation. Param provides "lazy reactive" APIs (depends(watch=False), bind(watch=False), rx(), etc.) that are meant to be introspected by a framework like Panel to build its own reactive workflow. It seems like Param does not document this at all and, in fact, lacks that kind of functionality. One example that comes to mind is the private and undocumented attribute _dinfo, which is accessed quite a bit internally in Panel for introspection.

• Look for cases similar to _dinfo in the main HoloViz code bases that uses Param (Panel, HoloViews, Lumen)
• Add public APIs for Param when needed to replace private functionality access
• Look for utilities that would be useful to upstream into Param (e.g. good candidates in https://github.com/holoviz/panel/blob/f9a72b70facb25ea3f327c29bb0fc0946ec9d5c8/panel/util/parameters.py)
• Update the documentation (update and/or add user guide, update API ref)

[jbednar]

Right; our current documentation for reactive features is highly focused on users, and the needs of an external library building on these features are quite different and so far not made explicit.

[MarcSkovMadsen]

Same need for whatever private information @param.output creates. For example used by Panels Pipeline.