The PJ/y unit is confusing people and needs explanation

## Describe the bug
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Capacity is models of this type is often (but not always) defined in and energy unit per year, e.g. PJ/y. It indicates the amount of capacity that can deliver 1 PJ over an entire year. 1PJ is 277.78GWh. As there are 8760 hours in a year, this is equivalent to a capacity of 31.710MW. So when users enter e.g. a capital cost value for a technology, if they are using PJ/y, they should enter the cost of 31.710MW of output capacity.

This needs to be explained.

## To Reproduce
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## Expected behavior
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Users have reported this issue re documentation section "9.3.6. Existing Sectoral Capacity", but in reality it is an issue throughout the documentation. To address this, we need an additional sub-section in section 9 of the documentation that just deals with units in MUSE (i.e. explains that it's flexible and users can choose what they want, so long as they are consistent), along with a description of how to convert between various units to help people who are not from an energy background.

I see in FAQ there is already something like this, but it needs a separate section within the Inputs bit.

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## Context

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