Improve BandMathX documentation

The documentation of BandMathX in the Cookbook is a bit confusing:

• There is the application documentation: the documentation is quite long but non exhaustive.
• There is the About BandMathX C++ API section. It contains information on the BandMathX syntax and C++ example. It contains a lot of details on advanced use of BandMathX, and describes additional functionalities beside MuParserX. This section comes from the Software Guide migration.
• Document bit manipulation as it should be used with the int operator

Most users of BandMathX are using the application, and will never open the C++ API section. I think the documentation should be refactored as follow:

• All the syntax related information should go in BandMathX's documentation.
• The About BandMathX C++ API section should be removed, and the C++ part should be moved to the examples (or not, there is already an example for the BandMathX filter).
• We could add an example of the BandMathX application in this recipe, to point out the differences with BandMath