Refactor apportioning kriged biomass and back-calculation doc pages

[leewujung]

This is from #216 and https://github.com/brandynlucca/echopop/pull/6:

There are currently repeated materials and notation differences between the current "Apportioning kriged biomass density" and "Apportioning back-calculated abundance" pages. I think we can refactor these into a single section with consistent notation and a more linear presentation, consisting of the following 4 sections:

• Overview with a flow diagram
• Symbol definitions (currently in the "Apportioning kriged biomass density" page)
• Apportioning kriged biomass
• Apportioning back-calculated abundance

Symbols to be made consistent:

• number proportions of fish samples (which may necessitate changes to the notation used for weight proportion)
• number of fish samples: $n$ (right now in the "Apportioning kriged biomass density" page $N$ is used)
• abundance: $N$ (this is already used in the "Apportioning back-calculated abundance" page)