Remove qtassistant and reduce size of mantiddocs package

[peterfpeterson]

The current mantid help system uses qtassistant to show the help documentation. This method of including the docs is always in agreement with the code. However, there are some problems with this approach:

1. The mantiddocs package is ~200MB. A large portion of this is because qtassistant wants the docs to be in a compressed archive and unpacked.
2. qtassistant doesn't support javascript. javascript support would allow rendering equations in the application (rather than converting them to png via latex) and allow for sphinx plugins.
3. The online docs are a separate build, with separate artifacts, from the offline docs

Describe the solution you'd like

Describe alternatives you've considered

qtwebview might be able to do it

Additional context