Include qt.console.frontend_widget in docs
Example of adding input transformers.

Closes gh-4709
Standardise some module docstring first lines
Fix header levels in technical description of config
Update inputsplitter docstring
Downplay the need for pyreadline on Windows.

Closes gh-4544
Squash a couple more Sphinx warnings
Merge pull request #4712 from takluyver/docs-shotgun-3

Various docs fixes

* Tweak exclude patterns, which were accidentally excluding qt.console.frontend_widget
* Added example of adding input transformers (#4709)
* Tidied up and added some module docstrings, now that we're using the first line in the API index
* Update the inputsplitter module docstring, which was rather out of date.
* Fix some header levels introduced in #4265
* pyreadline is apparently no longer mandatory on Windows (#4544)