Define and document sources and inclusion rules for oceans, seas, straits, channels and passages

[axelboc]

Fix #346.

Like for political entities, I've added a spreadsheet to list the water bodies and test/document the inclusion criteria. It contains three groups for now: seas, straits and channels/passages (seemed to make sense to group the last two together, as passages are closer to channels than straits in my mind). I haven't added continents and oceans to the spreadsheet since we don't need to apply any criteria to them.

Note that it might be a good idea to double check the >= 2 states columns for straits and channels/passages in case Wikidata returned any wrong or arguable results.

[axelboc]

Here are all the entities that need to be added to the deck:

Seas	Straits	Channels and passages
Andaman Sea Arafura Sea Baffin Bay Banda Sea Barents Sea Beaufort Sea Bering Sea Bismarck Sea Celebes Sea Chukchi Sea East China Sea Great Australian Bight Greenland Sea Gulf of Aden Gulf of Alaska Gulf of Guinea Gulf of Saint Lawrence Ionian Sea Java Sea Kara Sea Laccadive Sea Laptev Sea Molucca Sea North Sea Ross Sea Scotia Sea Sea of Okhotsk Seram Sea Solomon Sea Sulu Sea Timor Sea Tyrrhenian Sea Weddell Sea	Bab-el-Mandeb Strait of Magellan Strait of Gibraltar Singapore Strait Strait of Dover Palk Strait Strait of Hormuz Torres Strait Strait of Malacca	Mozambique Channel

And the ones that need to be removed:

• White Sea
• Denmark Strait

If the inclusion rules are confirmed, I'll create an issue to keep track of these additions/removals.

[aplaice]

On a cursory view (unfortunately, I currently don't have time for a thorough review, and likely won't this week), both the changes to CONTRIBUTING.md and the physical-entities.xlsx spreadsheet look great and in line with what we had discussed!

[aplaice]

I still haven't had time to properly review (i.e. double check everything in the spreadsheet), but that's not reason to delay merging even longer!

[axelboc]

Alright, merging now then! 🚀

I'll try to do a release next week.