Flowsheet documentation for RO w/ ERD

[kurbansitterley]

Fixes/Resolves:

RO w/ ERD flowsheet documentation for #1219

Summary/Motivation:

Add flowsheet documentation for RO_with_energy_recovery.py

Changes proposed in this PR:

• add doc .rst file
• add figures for each ERD configuration

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.94%. Comparing base (eef84c0) to head (e8c5b96).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1446   +/-   ##
=======================================
  Coverage   93.94%   93.94%           
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  Files         310      310           
  Lines       33206    33206           
=======================================
  Hits        31194    31194           
  Misses       2012     2012           

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[MarcusHolly]

2 minor things to address, but otherwise LGTM

[savannahsakhai]

LGTM

[lbianchi-lbl]

@kurbansitterley @adam-a-a FYI here's how I solved the merge conflict in the index page, let me know if something doesn't look right: