Safe c interop

[hnrklssn]

This adds documentation around __counted_by, __sized_by, API notes, and using lifetime attributes in C.
This is stacked on top of #980.

[rapidsna]

CC @egorzhdan @Xazax-hun @DougGregor

[rauhul]

Have you considered putting this documentation in a docc catalog?

[hnrklssn]

Have you considered putting this documentation in a docc catalog?

I have not! I can't speak for Gábor and Egor, but I simply kept writing in the established file. What are the benefits of a docc catalog? I haven't contributed to the website before, so I don't really know how things work

[rapidsna]

I remember the title was "Safe C++ and Swift interop" or something like that. If so we would also need to update it to include "C" as well.

[hnrklssn]

I'm having a bit of trouble deciding how to format the comparison between C and Swift; the narrow page size makes it hard to fit side-by-side comparisons. Following the approach used elsewhere on the page looks like this:

Code blocks cannot be used in markdown tables, so these have to be inline code snippets, which prevents formatting the code in a nicer way (as well as syntax highlighting).

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Using HTML tables we can get tables with code blocks. This gives us syntax highlighting, but unformatted it requires a fair bit of horizontal scrolling.

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With some formatting the situation improves, but they still don't fit without horizontal scrolling.

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We could instead skip the table and just use two separate code blocks before/after each other. Following the code formatting of the previous code blocks the C code looks like this, which works I guess, but doesn't look great right after each other imo.

[Xazax-hun]

Looks good to me, some nits inline.

[Xazax-hun]

I don't have a strong opinion on the formatting, looks like all approaches have pros and cons :/

[rapidsna]

+1 for the 3rd option, despite horizontal scrolling.