Added a section in docs explaining ReEDS uses HHV heating value conve…

[rrolph575]

…ntion.

Summary

Added a subsection under "Fuel Prices" section of documentation saying that the HHV convention is used for heating value in ReEDS. This is not explicitly stated in the docs and can be a source of confusion for users who are used to input values from IEA, EU, or manufacturers fuel prices.

Details to double-check

• [WCRD.13600.01.01.01 ] Charge code provided to reviewers

General information to guide review

• [ x] Zero impact on results of default case
• [x ] No large data file(s) added/modified
• [x ] No substantive impact on runtime for full-US reference case
• [x ] No substantive impact on folder size for full-US reference case
• [x ] No change to process flow (runreeds.py, reeds/core/solve/solve.py)
• [x ] No change to code organization
• No change to package requirements (environment.yml or Project.toml)

Did you use LLM tools (chatbot or copilot) in the preparation of this PR? If so, describe how

I did not use verbatim text but I used an LLM to help me in my research (which I double checked on other sources for correctness).