Add Calling conventions Doc

[dsouzai]

This information was only available internally; this PR adds it to the repo to aid with debugging.

[dsouzai]

@fjeremic could you please review?

[0xdaryl]

A couple thoughts from me:

• What's the reason to put Windows and Linux together in the same table? Just a space savings? Since this is a reference doc the length doesn't matter as much and I'd rather separate them. It will give us more freedom to add other commentary specific to that particular linkage later if desired (e.g., describing how memory arguments are passed and how the memory for those arguments is allocated) and might prevent some misinterpretations.

  Separating might also allow us to reorder the registers differently for each calling convention. For example, we could list all the integer parameter registers together and in passing order.

• Personally, I'm not really a fan of the emojis. I would prefer just a simple "V" or "P", but that's just me.

• If there are public documents for any of these calling conventions perhaps we could link to them, especially if they are harder to find (e.g., P and Z linkages)

[dsouzai]

@0xdaryl made changes suggested, could you take a look?

[andrewcraik]

Looking good

[dsouzai]

@fjeremic do you mind merging if you're satisfied as well?

[fjeremic]

Great initial doc. Let's check this in and we can improve if we think there is a need.

[dsouzai]

@knn-k do you mind creating the tables for arm / aarch64?

@janvrany do you mind creating the table for riscv?

[knn-k]

I opened #5308 for 32-bit Arm and AArch64.

[janvrany]

I opened #5313 for RISC-V