Update documentation & add usage for UpdateScriptRepository

Refs #9574
Implement usage for NormaliseByPeakArea

Also clarify a property description
Refs #9574
Remove unrequired testcleanup sections

Refs #9574
Add hidden toctree to index.rst

This squashes all of the warnings about most documents not in a toctree.
Refs #9574
Add usage examples to DeleteWorkspace.

Refs #9574
Add reference to t0 parameter in NormaliseByPeakArea.

Refs #9574
Add usage for Power algorithm.

Refs #9574
Update documentation of Mean and add a usage test.

Refs #9574
Improve CreateSingleValuedWorkspace docs and add usage tests.

Refs #9574
Add examples for LoadVesuvio.

Refs #9574
Usage examples for CalculateGammaBackground.

Refs #9574
Rebin parameters don't all apply to InterpolatingRebin

Also amalgamates code to accept single bin width as input rebin parameter.
Refs #9574
Usage example for InterpolatingRebin.

Also updated Rebin documentation to provide linkable content so that
InterpolatingRebin can refer to it.
Refs #9574
Centre out-of-line equations on their line.

The css that should do this in basic.css doesn't work with the bootstrap
theme as it has an extra div.body tag on the section containing
"text-align: center" so that it doesn't match in the bootstrap theme.
Refs #9574
Usage examples and doc cleanup for Polynomial correction.

Refs #9574
Package up the html help

Use the same location that the QtAssistant helpused to go.
Refs #9611
Switch from qtassistant to web browser ...

pointing at a local copy of the documentation.
Refs #9611
Remove qtassistant and wiki tools

No longer required now that the documentation is in the rst files.
Refs #9611
Fix broken category links.

Refs #9611
Use cmake variable to control packaging of offline documentation

It's not built by all platforms on incremental builds so it needs to be
turned off in these situations.
Refs #9611