Document workflows for intensity calculations

Both issue #243 and #244 suggest ways of calculating the mean and standard deviations within a spectrum, respectively:

> Oh and I guess the snippet above is now (v0.4) np.mean(extract_region(spectrum, reg).flux) or astropy.stats.sigma_clip(extract_region(spectrum, reg).flux, cenfunc=np.mean) if you want sigma-clipping

and

> For completeness, in v0.4 that would be np.std(extract_region(spectrum, reg).flux)

These do not rise to the level of needing dedicated analysis functions but (as suggested in the linked issues) should be documented.

Start by locating the documentation area for spectrum analysis and review issues #243 and #244 alongside the examples in this issue. Document workflows for calculating the mean and standard deviation of extracted flux, including the shown sigma-clipping option; done means users can find and follow these examples without dedicated analysis functions.

Written by the indexing model from the issue text.