[Docs] Added a calling convention summary document.

[al45tair]

Added a summary of x86-64 and ARM64 calling conventions, with links.

[al45tair]

@swift-ci Please smoke test

[CodaFi]

Could you convert this document to markdown? We're slowly trying to transition all the restructured text files from the legacy stuff there so they're readable on Github.

[benrimmington]

Should this replace the existing RegisterUsage.md document?
It is referenced by the Error Register section of the ABI Stability Manifesto.
Please also update the documentation index for any files added (or removed).

[al45tair]

@CodaFi This particular file is definitely better in ReStructuredText, because of the large tables. Markdown table support is frankly a bit rubbish (particularly in its ASCII form), and for anything more than a simple table you end up having to use HTML, which is… unpleasant. I don't understand why you think it wouldn't be readable on GitHub; it definitely is.

[al45tair]

@benrimmington

Should this replace the existing RegisterUsage.md document?

It could do, I suppose. It covers a bit more than just register usage, though, and I didn't really want to remove RegisterUsage.md because people will likely have linked to it. Maybe editing it to point at this instead would be OK? I'll do that and see if anyone objects.

It is referenced by the Error Register section of the ABI Stability Manifesto.
Please also update the documentation index for any files added (or removed).

Good idea.

[al45tair]

@swift-ci Please smoke test

[al45tair]

@CodaFi On GitHub, the current revision looks like this.

[al45tair]

@swift-ci Please smoke test

[al45tair]

@swift-ci Please smoke test