Add guidance on using the allometry functional unit testing

[glemieux]

Is your feature request related to a problem? Please describe.
Per discussion with @rgknox and @ckoven on NGEET/fates#586, we decided that we need to help users get familiar with the allometry tooling in https://github.com/NGEET/fates/tree/master/functional_unit_testing/allometry.

Describe the solution you'd like
Add a how-to page describing how to use the tool and add the page to https://github.com/NGEET/fates-users-guide/blob/main/docs/source/user/how-to.rst

Describe alternatives you've considered
No other alternatives considered. We should direct users with similar issues (from the fates issue board) to this new page.

Additional context
Add any other context or screenshots about the feature request here.

[glemieux]

@rgknox would you mind writing up the documentation for this?

[jenniferholm]

Let me know if you would like any explanation on this. I think I corrected this shrub allometry issue by reducing the fates_recruit_hgt_min parameter for shrubs, to something like ~0.2