first draft of clm6 documentation updates

[swensosc]

Description of changes

updates to ctsm documentation

update 2.5. Momentum, Sensible Heat, and Latent Heat Fluxes
add biomass heat storage -related text
update 2.25 Methane Model
add description of surface layer addition to transport equations
add 2.?? Hillslope Hydrology chapter

Specific notes

Contributors other than yourself, if any:

CTSM Issues Fixed (include github issue #):

• Resolves Review 2.26. Methane Model #3872

Are answers expected to change (and if so in what way)?

Any User Interface Changes (namelist or namelist defaults changes)?

Does this create a need to change or add documentation? Did you do so?

Testing performed, if any:
(List what testing you did to show your changes worked as expected)
(This can be manual testing or running of the different test suites)
(Documentation on system testing is here: https://github.com/ESCOMP/ctsm/wiki/System-Testing-Guide)
(aux_clm on derecho for intel/gnu and izumi for intel/gnu/nag/nvhpc is the standard for tags on master)

NOTE: Be sure to check your coding style against the standard
(https://github.com/ESCOMP/ctsm/wiki/CTSM-coding-guidelines) and review
the list of common problems to watch out for
(https://github.com/ESCOMP/CTSM/wiki/List-of-common-problems).

If this PR changes the CTSM web documentation, please see our contribution guidelines for documentation.

[samsrabin]

Thanks, @swensosc! Could you please fill out the PR template? Also, please describe in high-level terms the changes you're making—at least what chapters you're touching. Thanks!

[samsrabin]

Also:

• When you say "first draft"—are you expecting this PR to be merged as-is (after review), then you'll make further updates later? Or were you planning to make more changes to this PR? If the latter, please mark the PR as a draft so we know not to review it yet.
• Does/will this contain all changes needed to the methane chapter, thus resolving Review 2.26. Methane Model #3872?

[slevis-lmwg]

@swensosc do you feel this is ready for review?

[swensosc]

hi, sam, yes, I think it's ready.

…

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[slevis-lmwg]

I added this to my Sprint board to avoid losing track of it.
I hope to finish the review tomorrow.
So far it looks great.

[slevis-lmwg]

Thanks @swensosc for this work! Looks good. I will take care of the typo that I reported :-)

[slevis-lmwg]

@swensosc I consider this ready to merge to b4b-dev.
Let me know if you wanted to take a last look before I merge.