Add HVV convention in docs

[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).

[wesleyjcole]

A few comments:

• What do you think about moving this section to the fossil and nuclear technologies section where these things (fuel costs, heat rate) are first introduced?
• I don't think we need to explain the difference between HHV and LHV, or note locations where one is more common than another. I think the goal is just so people know we use HHV, which can be pretty short (a single sentence would probably be fine).
• In order to make it easier to identify changes to .md files, we prefer one sentence per line for any multi-sentence sections.
• Would you update the pull request title so that it fits on the single line?

[rrolph575]

I removed most of the text so that it is now just 2 sentences with now one sentence per line. I also moved those sentences to the section you recommended and agree it fits better there. I shortened the PR title.