Document bundled secondary tools/calculators

[k4kfh]

There are a number of useful "secondary" tools, accessible via the Tools menu at the top of the main window. These need their own segments of the documentation.

[ra3xdh]

Please wait a bit with this task. The filter design tools will be replaced by new RF design tool since the release 25.3.0. Old filter, attenuator, and power combining design tools will be removed. See ra3xdh/qucs_s#1432 for more details. I would suggest to ignore old tools, and write documentation for new tool.

The active filter design tool was documented for the old Qucs. See https://github.com/Qucs/qucs/tree/stale/spice4qucs_current/qucs-doc/tutorial/activefilter and https://www.researchgate.net/publication/275053492_Qucsactivefilter_-_Active_filter_synthesis_subsystem_of_Qucs The interface is identical, except it generates Nutmeg equations for Ngspice.

[ra3xdh]

RF layout is external tool developed by @thomaslepoix It has its own package and documentation and must be installed separately.