Codify 'missing content' practices for ports

[mqole]

mkanke brought up wanting to port funky-station/forky-station#26 but the PR contains recipes reliant on a preexisting construction material not found on Macro (metallic hydrogen).

We previously also ran into this issue with syndicate-ss14/macrocosm#3 as the guidebook for Grays contain Impstation-original prototypes (newport cigarettes and green cannabis) which are not found on Macro. In the case of the guidebook, it was very minor, so we replaced the prototype references with existing upstream prototypes which were similar enough.

This approach will however cause issues and merge conflicts for downstreams. mkanke noted this when it was suggested that we port the Funky PR and change references to metallic hydrogen to plasteel.

Per mkanke:

A - Hydrogen curbs are only commented out on Macro but left intact
B - Hydrogen curbs are fully removed but RSI folder is left so implementation later is easier
C - Hydrogen curb is commented out and RSI removed so Micro's know its an option they can port later
D - Hydrogen curbs are fully removed from the Macro port altogether

Personally, especially in this case I would not say changing would be good as it would mean that it would introduce another brand new version when Forky pulls from Macro aka the plasteel versions and would opt to either comment out or outright remove them.
I could see it being something such as "If the object can reasonably be replaced with a similar existing item use that otherwise remove it from the upstream" being an approach

Once our approach to this has been decided it should be codified in our docs PR guidelines.